The participants were part of a longitudinal birth cohort study, Steps to the Healthy Development and Well-being of Children (the STEPS study), in the Hospital District of Southwest Finland (Lagström et al., 2013). The Finnish Ministry of Social Affairs and Health and the Ethics Committee of the Hospital District of Southwest Finland approved the STEPS Study in 2007. Participants were enrolled from an eligible cohort of 9,811 Finnish- and Swedish-speaking mothers at maternity clinics during pregnancy or on the delivery ward at birth. From the eligible cohort, 1,797 mothers provided written consent to participate in the study and were informed of the possibility to withdraw at any time. The recruited children were born between January 2008 and April 2010.

Inclusion criteria for the present study were language data in Finnish for the child at both ages 13 and 24 months. Children with language data in Swedish or both Finnish and Swedish were not included, either because Finnish was not the child’s first language or its status in the family was unknown. Exclusion criteria were children born preterm (<259 days) or with missing gestational data, and children with diagnosed impairments possibly affecting language development (i.e., epilepsy, cleft palate). Families with a mother with another home language than Finnish but who had completed the language questionnaires in Finnish were also excluded. The final sample was 369 boys and 350 girls (N = 719, two twins). Figure 2 summarizes the enrolment procedure.

Expressive vocabulary growth between 13 and 24 months of age was assessed using the parental report of the Finnish MacArthur Communicative Development Inventory (CDI): Words and Gestures (CDI-I) at the child’s age of 13 months, and Words and Sentences (CDI-T) at age 24 months (Lyytinen, 1999; Test information cf.; Fenson et al., 2007). The parents could complete the questionnaires on the study’s website or on paper returned with a stamped envelope. A new questionnaire was sent if there was no answer within 2 weeks. The evaluation of vocabulary production by parental reports has been found to correlate highly with standardized testing (Fenson et al., 2007) and has been considered a valid tool to examine early vocabulary (Feldman et al., 2005; Korpilahti et al., 2016; Thal et al., 1999).

The outcome measure was vocabulary growth between the ages of 13 and 24 months in the various lexical categories comprised in the measured expressive vocabulary. The Finnish CDI-I and CDI-T inventories consist of 19 and 20 semantic or lexical categories, respectively, which were analyzed in the following lexical categories: sound effects and animal sounds (13, 13 items), common nouns (207, 293 items), people (16, 24 items), games and routines (18, 22 items), action words (60, 106 items), descriptive words (26, 54 items), time words (8, 12 items), pronouns (8, 24 items), questions (7, 8 items), prepositions (11, 20 items), amount (6, 9 items), and particles (10 items only in the CDI-T). Growth between the measure points 13 and 24 months was analyzed for all lexical categories except particles, which was only available in the CDI-T. We analyzed the vocabulary in number of words at ages 13 and 24 months. To be comparable, the growth between 13 and 24 months was analyzed in percentage words of the total vocabulary in each lexical category, as numbers of words in the lexical categories differ between the CDI-I and CDI-T. A function factor was constructed of five items measuring different kinds of grammatical categories. These were time words, pronouns, questions, prepositions, and amount. All these categories express different kinds of grammatical function words that start to develop in the latter part of the second year.

The families completed parental and child questionnaires about demographic factors such as educational and occupational background, family structure, health history, and day care attendance at ages 13 and 24 months. The questionnaires were answered at gestational weeks 10–15 by the mothers, at gestational weeks 20 and 30 separately by both parents, and when the child was aged 13 and 24 months by one of the parents. Mothers recruited on the delivery ward completed the first questionnaire at that time.

Independent child factors analyzed in the study were being the firstborn and reported day care attendance at 13 and 24 months of age. Family factors were parental educational level, occupational level, and a family history of late onset of speech. All child and family variables were analyzed separately for lexical growth in boys and girls. The firstborn variable was analyzed as dichotomous (yes or no). The child’s attendance in day care outside the home was asked for at 13 and 24 months and analyzed as dichotomous variables (yes or no). At age 13 months, only 21.1% of the children were in day care outside the home compared to 53.15% at age 24 months. Educational level was analyzed in four categories for the mothers and fathers: no occupational education (n = 101 [14.5%] and n = 196 [28.7%], respectively), college degree (n = 146 [20.9%] and n = 173 [25.3%], respectively), lower university degree (n = 233 [33.4%] and n = 164 [24.0%], respectively), and higher university degree (n = 217 [31.1%] and n = 150 [22.0%], respectively). In cases where the mother or father had chosen the alternative “other education” in the questionnaire, this was regarded as a missing value (n = 22 and n = 36, respectively), as there was no way to know the level of the education the parent had participated in. The occupational status of the mothers and fathers was analyzed in three categories: low (including farmers, construction, process or transport workers, n = 55 [8.7%] and n = 183 [31.5%], respectively), medium (including office and service workers, n = 187 [29.5%] and n = 70 [12.0%], respectively), and high (including managers, specialists and professionals, n = 391 [61.8%] and n = 328 [56.5%], respectively). A family burden of late onset of speech was analyzed as a sum variable of parental self-reports of late onset in their own speech, in a sibling’s speech or in the speech of a near relative and was categorized as a dichotomous variable (yes or no). There were no significant differences in background variables between boys and girls. It is therefore feasible to assume that the two groups were comparable. See Table 1 for a descriptive overview of the participants.

The analyses for the present study were carried out using the IBM Statistics SPSS 25–26 and Mplus 8.0 software with Maximum Likelihood estimator (Muthén and Muthén, 1998–2011). Missing data on the dependent variables (0.1–0.3% per item) were handled with the Expectation Maximization procedure. In the descriptive analysis of the present study, mean and standard deviation (SD) for continuous variables and number and percent for categorial variables were used to describe the study participants. The differences in the mean size of lexical categories and in vocabulary growth of lexical categories between boys and girls were analyzed using an independent two tailed t-test. Pearson´s correlation analysis was used in comparing the relationship between growth in the lexical categories in boys and girls. p-values of less than 0.05 were considered statistically significant.

To test the hypothesized model of differences in early vocabulary growth of lexical categories, SEM analysis was performed (See Figure 1). SEM provides the possibility to use multiple indicators to represent a latent variable leading to a reduction in measurement error, analyses of multiple dependent variables simultaneously, flexible handling of missing values on the dependent variables, and testing coefficients across multiple between-subjects groups (e.g., Bollen, 1989; Byrne, 2012). Analyses were conducted using, first, confirmatory factor analysis (CFA) to examine the factor structure of the latent outcome variable, and second, SEM to assess the regressions, including the CFA models and observed variables. In all cases, factors were allowed to correlate, and errors were assumed to be uncorrelated. The fit of the models was evaluated by the chi-square test statistic and fit indices including root mean square error of approximation (RMSEA), standardized root mean square residual (SRMR), Tucker-Lewis index (TLI), and comparative fit index (CFI). The following cutoff values were applied: RMSEA values under 0.08 and TLI and CFI values preferable over 0.95 (e.g., Hu and Bentler (1999)) but acceptable if over 0.90 (e.g., Kline, 2011; Metsämuuronen, 2009). While the ratios of the chi-square statistic and degrees of freedom were carefully considered, statistical significance of the chi-square value alone was not interpreted to indicate an inadequate fit (Hu and Bentler, 1995; Byrne, 2012).

To examine the extent to which possible differential item functioning of the outcome factor “function words” may affect the group differences in focus in the present study, a comparison of multigroup CFA (MGCFA) models across the child’s sex was performed, starting with the less restricted model to the more constrained model as suggested by Brown (2015). We applied a hierarchy employing two levels of measurement invariance: configural and metric. The metric invariance is considered sufficient for the purposes of the present study, as it meets the prerequisite to examine structural relationships between variables (e.g., Kline, 2011). In determining the invariance of nested models, we examined the differences in multiple fit indices including the RMSEA, CFI, and TLI. In comparing the metric model with a less restricted configural one, we used the critical values of 0.01 and 0.015 for the differences in CFI and RMSEA, and 0.022 for TLI (e.g., Chen, 2007). To examine the between-group differences in the hypothesized regressions, a multigroup SEM (MGSEM) was applied. While the invariance of the regressions would indicate similar effects of background variables on language development in boys and girls, the non-invariance confirms the differential associations.

There was a large variation in total vocabulary size at 13 and 24 months of age in and between boys and girls. Mean vocabulary size at 13 and 24 months of age in boys was 7.8 (SD = 10.9, Range = 0–116) and 262.2 (SD = 168.1, Range = 4–593), respectively, and in girls 11.8 (SD = 22.8, Range = 0–297) and 340.3 (SD = 152.5, Range = 4–595), respectively. Total vocabulary growth between the ages of 13 and 24 months in boys was mean% 42.0 (SD = 27.5) and in girls mean% 54.1 (SD = 24.6).

Mean sizes of the various lexical categories varied widely between the children. Mean number of words in the category of common nouns at age 13 months ranged from 0 to 71 in boys and 0 to 179 in girls. Forty-two percent of the boys and 35% of the girls had no words in the common noun category. At 24 months of age, the variation in vocabulary size of nouns was still large, varying from 0 to 291 words in boys and 0 to 293 words in girls. At 2 years of age, 1.7% (n = 7) of the boys and 0.6% (n = 2) of the girls had no noun in their vocabulary. The girls had a significantly larger vocabulary size than boys at 13 months of age in four of the lexical categories: sound effects and animal sounds, common nouns, people, and games and routines. At 24 months of age, the girls outperformed the boys in all lexical categories (p < 0.001).

The percentual growth of vocabulary between the ages of 13 and 24 months was mean% 0.42 (SD = 0.27) in boys and mean% 0.54 (SD = 0.25) in girls. Vocabulary growth in lexical categories varied substantially in and between the categories. Some children had fewer words in some categories at 24 months than at 13 months of age, which explains the negative numbers in the ranges (Table 2). Girls outperformed boys in vocabulary growth in all lexical categories except sound effects and animal sounds (p < 0.001). However, the effect sizes were small to moderate (d = −0.25 to −0.47).

The result of the Pearson correlation indicated a significant positive association (p < 0.001) between lexical growth in the lexical categories in boys (r = 0.237–0.932) and in girls (r = 0.196–0.921), except for associations between sound effects vs pronouns and questions (r = 0.182, p = 0.001 and r = 0.164, p = 0.002, respectively) (Table 3). The correlations between sound effects and the other lexical categories were weak in both boys and girls (r = 237–447 and r = 182–365, respectively).

Before predicting study outcomes, we examined the structure and invariance of the function factor across the sexes. The internal consistencies of the scale varied between 0.923 ≤ α ≤ 0.937, showing excellent measurement reliability (e.g., Nunnally and Bernstein, 1994). With regard to normality, the study items’ univariate distributions were all within a reasonable range (skewness ±2, kurtosis ±7; see Curran et al., 1996). In addition, across both sexes the freely estimated factor loadings were significant (p < 0.001) and the coefficients were salient, ranging from 0.808 to 0.930. Table 4 shows the tests for measurement invariance across the child’s sex. The first step involved testing the same factor structure with boys’ and girls’ empirical covariance matrix separately. This produced very well-fitting models in each group.

A configural invariance model also showed an excellent fit to the data (Table 4, model M1). Further, constraining corresponding factor loadings to be equal across the sexes produced the metric invariance model which exhibited a good overall fit (M2 in Table 4). The minor changes in GFIs indicated full metric invariance of the function words scale across the study groups.

The initial structural model including the hypothesized associations between independent predictors and vocabulary growth outcomes (see Figure 1) demonstrated a well-fitting multigroup model to the data: χ² (128) = 285.13, RMSEA = 0.071, CFI = 0.976, TLI = 0.947, SRMR = 0.018. However, constraining the regression coefficients to be equal across the sexes resulted in a misfit of the data. This suggests that child and family factors affecting early vocabulary growth vary between boys and girls. Figures 3, 4 report the findings of the fully estimated models separately for both groups. Only statistically significant associations are included in the models.

In boys, a family burden of late onset of speech predicted less vocabulary growth in all lexical categories except for the category of sound effects and animal sounds. However, this category was the only lexical category with larger growth in those boys attending day care at 24 months of age. In girls, being firstborn predicted positive growth in descriptive and function words. In girls, attending day care predicted larger growth of words in the categories sound effects, nouns, people, and games and routines. (Figures 3, 4). However, the amount of explained variance was small.

The largest lexical group in both boys and girls at 13 months of age was common nouns, followed by sound effects, people words, and games and routines. The predominance of nouns in early vocabulary acquisition as the vocabulary size increases is congruent with earlier studies (e.g., Bates et al., 1994; Caselli et al., 1995; Stolt et al., 2008). At 24 months of age, nouns still formed the largest group in the vocabulary of both boys and girls, followed by action words, descriptive words, and games and routines. When the vocabulary reaches over 50 words, action and descriptive words start to develop (Wehberg et al., 2007), as was similarly observed at group level in both boys and girls at 24 months of age in our study. We found that the individual variability of vocabulary size within the lexical categories increased with age, as has been suggested in earlier studies with English, Italian, Slovenian, and Estonian children measured with CDI (Caselli et al., 1999; Marjanovič-Umek et al., 2016; Schults and Tulviste, 2016). The girls outperformed the boys at age 13 months in the lexical categories of sound effects, common nouns, people, and games and routines words, and at 24 months in all categories. This is comparable with the results of Schults and Tulviste (2016), who found significant differences between boys and girls in vocabulary size at ages 1;2 and 1;4 (N = 903) in all categories except descriptive and function words. None of the children in the present study reached the maximum ceiling of available words in a category at 13 months of age, whereas at 24 months the ceiling of all lexical categories was reached by some of the girls. Among the boys, some reached the ceiling in all categories except nouns, where the ceiling was not reached.

Considering the vocabulary growth, calculated in percentage, in the lexical categories between 13 and 24 months of age in boys and girls, the same five categories were the fastest growing but in a slightly different order. In boys, sound effects showed the largest growth, followed by games and routines, nouns, action words, and prepositions, whereas in girls, games and routines was the largest growing category followed by sound effects, nouns and action words and prepositions. As for the growth size in the various lexical categories, the girls outpaced the boys in all lexical categories except for sound effects. The discrepancy between total vocabulary size in boys and girls has been shown to increase up to 30 months of age in a cross-linguistic study with ten different European languages (Eriksson et al., 2012). The same discrepancy should apparently be found in the size of and growth within the lexical categories, which was confirmed in the present study. The correlations between growth in the lexical categories were strong, except between sound effects and the other categories, where the correlation was small or medium-sized. The category sound effects and animal sounds is perhaps the one that differs most from the others, as it is large and transitory in early expressive vocabulary development and paves the way to more conventional words (Caselli et al., 1995).

A family burden of late onset of speech predicted in boys a smaller growth in all lexical categories except for sound effects and animal sounds. This is in line with our hypothesis and with previous studies suggesting an effect of late onset of speech in the family. The result also supports the second hypothesis of different effects in boys and girls. The effect of a family history of late onset of speech and language problems on early language development has been reported in many previous studies (Bishop et al., 2012; Keegstra, 2007; Reilly et al., 2010; Zambrana et al., 2014; Zubrick, 2007). Reilly et al. (2010) and Bishop et al. (2012) reported that a family history of speech and language problems in children with late onset of speech at 18–24 months of age predicted more persisting language difficulties compared to children without this background. In the studies mentioned above, the focus has been more on the relation between a family history of late talking, reading, and writing difficulties and language impairment. However, in the present study, the parents only reported a late onset of speech of their own, in siblings, and/or in the near family. Language impairments or problems with reading and writing were not accounted for. A parental report of late onset of speech in the family, without information on diagnosed language problems, was enough to predict slower growth in lexical categories between 13 and 24 months of age in boys. This confirms that already at this early age we can detect potential later language problems, which is of significance as it has been suggested that almost 50% of late talking children aged 2 years still have poorer language skills at 10 years of age (Armstrong et al., 2017). The only lexical category not affected by a family burden of late onset of speech in the present study was that of sound effects and animal sounds. This category is often large in the beginning of vocabulary growth (the first five words) and contains sounds resembling word forms which later give way to more conventional words (Caselli et al., 1995; Schults and Tulviste, 2016; Wehberg et al., 2007). A possible explanation as to why this category was not affected by a family burden of late onset of speech could be its role as a more transitional category in the lexical development. Laing (2014) suggests that these first sound-resembling words, called onomatopoeic words, serve as a bridge to more conventional words. In the present study, paternal late onset of speech affected boys but not girls. One reason for this could be that heritability for expressive vocabulary has been found to be more than two times higher in boys than in girls (Van Hulle, et al., 2004).

In the present study, neither educational nor occupational status of the parents predicted lexical growth in boys or girls, which contradicted both of the hypotheses. The result was contrary to earlier studies by Fernald et al. (2013), Gilkerson et al. (2018), and Hart and Risley (1995), but confirmed the studies by Henrichs et al. (2011) and Kuvač-Kraljevic´ et al. (2021). The lack of effect of high education or occupational status on early vocabulary growth can reflect cultural differences in educational system and how this has been measured in different studies. The studies of Fernald et al. (2013), Gilkerson et al. (2018), and Hart and Risley (1995) were performed in English-speaking communities, while for example those of Henrichs et al. (2011) and Kuvač-Kraljevic´ et al. (2021) were conducted in European countries. However, Lankinen et al. (2018) found high education of Finnish fathers to be associated with expressive vocabulary size at 24 months of age. In our study, over 60% of the participants’ (boys and girls) mothers had at least a lower university education, and at least 45% of the fathers. The occupational level was also high in mothers and fathers (over 60 and 55%, respectively). This should be taken into consideration when interpreting the results, as the large percentage of highly educated parents with advanced occupational status could have skewed the results. It is also possible that the effects of educational and occupational status would have come across through process variables, such as the habit of reading to the child, if they had been analyzed in this study. Vocabulary growth in lexical categories in both boys and girls were equally unaffected by the parents’ educational or occupational level. This contradicts the studies of Barbu et al. (2015) and Lankinen et al. (2018), where early language in boys was more affected by SES factors than in girls. The reason for this could be the high educational and occupational level of both parents and that the differences would be more obvious in low-SES families, as suggested by Barbu et al. (2015).

Being the firstborn predicted larger vocabulary growth in the lexical categories of descriptive and function words in girls, which was partly in line with our first hypothesis of the advantage of being firstborn, and in line with our hypothesis of different effects in boys and girls. There are indications that mothers generally talk more to their daughters than to their sons, even if both parents seem to talk somewhat more to their firstborn sons (Gilkerson and Richards, 2009). The vocabulary growth in boys was not affected by being firstborn or not. One explanation could be that boys lag behind girls in grammar development at this age and cannot take advantage of heard speech. Another explanation could be that even if firstborn boys hear more talk than those born later, the language addressed to them is more restricted than what girls hear (Gilkerson and Richards, 2009). Previous studies have suggested that firstborn children have an advantage over those born later (Hoff-Ginsberg, 1998; Urm and Tulviste, 2016). However, those studies focused on total vocabulary size, whereas in our study, only the growth in descriptive and function words was predicted by birth order. Descriptive and function words emerge more slowly and do not increase until the latter half of the second year, as the total vocabulary needed for developing grammar expands (Bates et al., 1994; Caselli et al., 1995; Marchman and Bates, 1994; Stolt et al., 2008; Wehberg et al., 2007). This was also found in our study, where descriptive and function words (time words, pronouns, questions, prepositions, and amount) were few at 13 months of age in the girls’ vocabulary but had increased at 24 months. The growth rate in the categories of descriptive and function words showed a substantial increase between 13 and 24 months of age. As it has been suggested by Hoff-Ginsberg (1998) that both vocabulary and grammar development are more advanced in firstborn children, it could be that of the whole vocabulary, descriptive and function words are the grammar-building blocks that are most influenced by being born first. The language the mothers use with their firstborn children is, furthermore, more advanced, with longer sentences and fewer questions, than language used with those born later Hoff-Ginsberg (1998), which could also explain the predictive function of being firstborn on the growth of descriptive and function words.

Day care attendance at age 24 months predicted vocabulary growth positively in the lexical category of sound effects in both boys and girls. However, only in girls did day care attendance predict vocabulary growth also in nouns, people words, and games and routine words. This was in line with our hypothesis of different effects in boys and girls. The results were not in line with our hypothesis that day care attendance does not predict vocabulary growth, and they contradict the results of Cadime et al. (2018), where no effects of time spent at day care were found in vocabulary size in any of the lexical categories at ages 16–23 or 24–30 months in Portuguese children. Cadime et al. (2018) used a broader distribution of lexical categories, with social terms (sound effects and animal sounds, people words, games and routines), nouns, predicates (verbs, descriptive words), and function words. In the present study, the growth of sound effects was greater in both boys and girls attending day care at 24 months of age compared to children not attending. This is the lexical group of word forms resembling animal sounds, car sounds, etc., and which will for the most part develop into conventional words towards the end of the second year (Caselli et al., 1995). One could argue that this is a category mostly used in play and group activities in day care, and that at home other categories are used more when talking to the child. This is, however, not applicable, because vocabulary growth in girls in day care at 24 months was also found in the lexical categories of nouns, people words, and games and routines. The outcome furthermore somewhat contradicts the results of Stolarova et al. (2016), who found a larger vocabulary in German girls under 2 years of age not attending day care outside the home. There is evidence that spending a great many hours in day care at a young age can negatively affect vocabulary size (Urm and Tulviste, 2016). As the vocabulary size of girls is more advanced than that of boys (Andersson et al., 2011; Eriksson et al., 2012), one explanation could be that girls attending day care at 24 months of age have a greater capacity than boys of the same age to take advantage of the language spoken in group situations. We could also assume that the speech children hear at home is more child-directed than at day care, where instructions are given to a group. There is in fact evidence that child-directed speech is associated with larger vocabulary size (Hart and Risley 1995; Rowe, 2008; Weisleder and Fernald, 2013), which could explain why attending day care only predicted the growth of sound effects in boys. Sound effects are usually onomatopoeic words which are possibly much used in play between young children and not so much in child-directed speech at home. Girls with a stronger vocabulary can presumably develop vocabulary also in day care settings without speech directed specifically at them. Among the girls there were also lexical categories that predicted no growth in day care attendance at 24 months of age. The categories affected by day care were sound effects and animal sounds, nouns, people words, and games and routines, which were already more developed at 13 months of age in the girls. All these categories, except for people words, belonged to the four largest developing categories between 13 and 24 months of age. This could suggest that the focus and language used in day care at this early age is related more to the first developing categories of sounds, people, and play words and nouns than to action, descriptive, and function words.

In the present study, the predictive value of day care attendance “overrides” the educational and occupational status of the parents in relation to early vocabulary growth. This is a surprising result. Day care in Finland falls under early childhood education and care legislation to secure lifelong learning, starting from day care. For each child attending day care, educational plans are created, including individual goals for the development of the child (Ministry of Education and Culture). Of the children in the present study, only 53% were in day care at 24 months of age. It has been suggested in international studies that children of parents with a higher educational level attend fewer hours at day care at a younger age (Urm and Tulviste, 2016), but also that children attending day care in the first year of life have parents who are more highly educated (de Hoog et al., 2014). As discussed above, it is possible that the effects of highly educated parents come across other variables such as day care attendance. Formal day care arrangements and the use of these services varies greatly in different countries. Only in Europe is there a large discrepancy in provision and use of formal day care services at the ages of 0–2 years, where 73% of Danish children attend day care compared to only 2% of Czech and Polish children (European Commission, 2009).

It can also be argued that cultural differences in environmental factors (such as day care arrangements, educational systems) influence and cause variations in the results of different studies. However, Eriksson et al. (2012), Bleses et al. (2008), Caselli et al. (1995), Braginsky et al. (2019), and Kuvač-Kraljevic´ et al. (2021) demonstrate in their studies including language societies inside and outside Europe that cultural differences do not supersede the basic process in early vocabulary development.

The results of the present study suggest that lexical growth is affected differently by child and family factors as a function of the child’s sex. Various factors predicted the growth in lexical categories in boys and girls in diverging ways. As there were no significant differences between the background variables in boys and girls, the differences we found in lexical growth were analyzed as a function of the child’s sex. These findings are in line with the hypothesis that there would be different effects of child and family factors on the development of lexical categories in boys and girls, and with previous studies by Barbu et al. (2015) and Lankinen et al. (2018) where sex differences in language development have been found to relate to family factors.

One explanation for the differences in how vocabulary development was predicted by background factors in boys and girls could be different susceptibilities in boys vs girls to the factors influencing early language development. It has been suggested in twin studies that early language development is influenced differently by biological and environmental factors (Galsworthy et al., 2000; Van Hulle, et al., 2004). There is a possibility that when analyzing boys and girls together in relation to language development and the factors affecting it, factors that would enlighten the gender variations in early language development are overlooked. This variation as a function of the child’s sex is important and needs to be studied more, as it brings new perspectives to early language development and is thus of significance in supporting early vocabulary development and children at risk.

A strength of the present study is the prospective longitudinal birth cohort design. Moreover, it presents new detailed information on the early vocabulary growth of lexical categories and how vocabulary growth is predicted by background factors as a function of the child’s sex. However, there are some limitations that need to be considered when interpreting the results. The effect sizes were small, meaning that the studied factors explained only part of the vocabulary growth in lexical categories between the ages of 13 and 24 months. This was true of both boys and girls. In the present study, vocabulary size at 13 months of age was not considered in the factor analysis; it could have added the aspect of where in the vocabulary development the child was at the beginning of the growth period. For some children with a larger vocabulary size, the growth could have continued on the same trajectory. On the other hand, for a child with a larger vocabulary already at 13 months, the growth may not have seemed as big as for a child with less vocabulary in the beginning. Another restriction was that the date of enrolment in day care was not asked for. This limited the interpretation of the effects concerning day care attendance, as we lacked detailed information on how long the child had attended day care.