With the accelerated pace of work and changes in living environments, the incidence of infertility is increasing annually. According to information released by the Department of Maternal and Child Health, National Health Commission, the incidence of infertility of men and women in China is 7%–10% (1). Several causes have been linked to infertility. Studies have shown that 25%–50% of infertile women suffer from endometriosis, 30%–50% of whom also suffer from infertility (2). Moreover, the implantation and pregnancy rates of in vitro fertilization (IVF) in endometriosis-related infertile patients are lower than those in tubal factor infertility patients (3). This may be related to the damage of endometrial receptivity and the abnormal contraction of the endometrium and its connected myometrium. Studies have shown that there is a high level of oxytocin (OT) in the serum of endometriosis (4). Previous studies have shown that OT acts on oxytocin receptor (OTR) in the uterine junction zone, activates the Ca²⁺ sub channel and releases Ca²⁺, thus achieving the effect of uterine smooth muscle contraction, and also stimulates the production and release of arachidonic acid and prostaglandin F_2a (PGF_2a), thus stimulating uterine contraction, which may be the main reason for the strong peristalsis and dysperistalsis of endometrium in endometriosis (5). Accordingly, inhibiting uterine contraction has become an important method for increasing the implantation of embryos in patients with endometriosis and reducing pregnancy loss.

Phloroglucinol is a non-atropine, non-papaverine pure smooth-muscle antispasmodic, which can reduce abnormal contraction of uterine smooth muscle, thereby helping to evade the risk of abortion (6, 7). Considering that age is an independent factor affecting the success rate of embryo transfer, the effect of administering phloroglucinol to endometriosis patients at different ages, before and after frozen embryo transfer on their clinical pregnancy outcomes, was retrospectively analyzed herein to provide a clinical basis for improving the assisted pregnancy outcomes among these patients.

To date, more than 8 million babies have been born worldwide as a result of IVF treatment (11). However, in the past four decades, the clinical outcomes of reproductive technology-assisted pregnancy have been unsatisfactory, with the live birth rate per cycle being only approximately 30% (12). Good embryo quality, endometrial receptivity, and synchronization of the endometrium and embryo development can provide the necessary conditions for the successful implantation of an embryo (13). If the uterine environment is abnormal, two-thirds of all pregnancies will fail (14).

Endometriosis is a common gynecological disease in women of childbearing age, and 30%–50% of women with endometriosis suffer from infertility (2), which may be related to impaired endometrial receptivity resulting from inflammation and immune abnormalities brought on by endometriosis itself, ultimately resulting in the abnormal contraction of the myometrium (5). Studies (15) have confirmed that ascites from patients with endometriosis contained high concentrations of prostaglandins (PGs), which stimulate uterine contraction and lead to abnormal tubal peristalsis, causing the arrival time of the fertilized egg in the uterine cavity to be out of sync with endometrial development, thereby affecting the implantation of fertilized eggs. In addition, during the embryo implantation period, ultrastructural changes occur in the endometrium, such as a lack of the expression of key factors, e.g., integrin, selectin, and cadherin, resulting in decreased endometrial receptivity. Under normal circumstances, the endometrium can generate endometrial peristaltic waves with non-synchronous contraction of the submucosal layer of the endometrium. The frequency of uterine contraction will be gradually reduced after ovulation and will steadily disappear during the implantation window period, which is conducive to the adhesion, positioning, and implantation of the embryo in the uterine cavity (16). However, during IVF-assisted pregnancy, some researchers have observed that a frequency <3 times/min of uterine contraction occurred less than 25% before embryo transfer but could reach 34% >5 times/min (17). Moreover, studies have found that about one-third of patients will have uterine contractions with a frequency of more than 5 times/minute on the day of embryo transfer, while too frequent uterine contractions will push the embryo to the fallopian tube or cervix, or even push the embryo out of the uterine cavity, leading to pregnancy failure (18, 19). As such, excessive uterine smooth muscle contraction and abnormal endometrial movement modes and frequency can lead to embryo implantation failure, early pregnancy loss, and even ectopic pregnancy (20).

Based on the above, inhibiting uterine contraction has become an important means of increasing embryo implantation and reducing pregnancy loss in infertile patients with endometriosis. At present, commonly used uterine contraction inhibitors include progesterone, oxytocin antagonist, and PG antagonist (21). Phloroglucinol is a myopathic non-atropine non-papaverine smooth muscle antispasmodic with 1,3,5-trihydroxybenzene as the active ingredient (6), which has no anticholinergic effect, no effect on cardiovascular function, does not cause hypotension, has no teratogenicity, mutagenicity, or carcinogenicity, and no adverse effect on embryo and fetal development (22). In one study, researchers administered 80 mg phloroglucinol intravenously twice a day to IVF-ET-assisted pregnancy patients up to 5 days before embryo transfer, trusting that it could reduce the uterine contraction of patients and increase the clinical pregnancy rate (23). Another study posited that repeat visits for medication are inconvenient for patients; therefore, only 80 mg of phloroglucinol was given intravenously before the embryo transfer, which also led to a good clinical pregnancy outcome (24).

In the above studies, phloroglucinol was only given for a single or multiple times during the embryo transfer period, but according to the law that uterine contraction drives endometrial peristalsis, in the natural state, with the development of follicles, the endometrial peristalsis from the cervix to the fundus increases, and reaches the peak before ovulation, which is conducive to sperm delivery. After ovulation, the peristalsis of endometrium is significantly reduced, forming a quiet and stable environment, which makes it possible for embryo implantation (25). Moreover, in this study, it was observed that endometrial peristalsis in endometriosis patients began to increase on the day of progesterone application, and some patients had uterine contraction symptoms within one week after embryo transfer, such as abdominal distension and dysmenorrhea, especially in the afternoon. Therefore, we gave continuous uterine contraction inhibition during the period of frequent endometrial peristalsis, and started to give phloroglucinol intramuscular injection in the afternoon of progesterone application day, once a day, until 7 days after embryo transfer. During embryo transfer, the dosage of phloroglucinol is still not uniform. Considering that phloroglucinol has a fast onset, short half-life, and the half-life of blood concentration is about 15 min, the blood concentration decreases rapidly within 4 h after administration, and then slowly decreases. After 48 h, there is only a small amount of drug residue in the body (26). In addition, previous literature reported that phloroglucinol 40 mg was injected intramuscularly, it also has a positive effect on improving the clinical pregnancy outcome of patients with repeated embryo implantation failure (27). Therefore, based on the principle of low dose, safety and effectiveness, we tried to use phloroglucinol at a dose of 40 mg.

The results of this study showed that continuous low-dose phloroglucinol pretreatment before and after embryo transfer could significantly improve the implantation, clinical pregnancy, and live birth rates of endometriosis patients by inhibiting excessive endometrial peristalsis and uterine smooth muscle contraction from ovulation to mid luteal phase, and the results were consistent with existing studies (23, 24). Moreover, the results of a meta-analysis in 2019 (7) showed that phloroglucinol had advantages in the overall response rate to the threat of abortion, the time to relieve uterine contraction, and the symptoms of uterine contraction; the same results were obtained in the current study. In addition, during phloroglucinol intervention, these patients had no complaints and experienced no allergic reactions.

Considering age as an independent factor affecting the success rate of pregnancy, the age of 35 was used as a boundary for dividing the patients with endometriosis into <35 and ≥35-year-old groups, respectively. The results showed that in young patients, continuous small-dose administration of phloroglucinol before and after embryo transfer could significantly improve pregnancy outcomes. However, for the older patient group, although the embryo implantation, clinical pregnancy, and live birth rates after phloroglucinol pretreatment were higher than in the control group, no statistical difference was observed. This indicated the presence of several factors affecting the blastocyst implantation in older patients, and phloroglucinol could not improve the pregnancy outcome in these participants with endometriosis. In addition, phloroglucinol pretreatment did not reduce the abortion rate in older patients under the influence of an increased risk of fetal chromosomal abnormalities. Existing studies have shown that phloroglucinol pretreatment before embryo transfer could effectively inhibit the occurrence of ectopic pregnancy (23). In this study, although patients of different ages accounted for a relatively high proportion of the embryos in the cleavage stage of embryo transfer, the rates of ectopic pregnancy were low, and no significant differences were observed. It was considered that phloroglucinol may inhibit excessive peristalsis of the endometrium, thereby reducing the occurrence of an ectopic pregnancy.

In summary, the infertility caused by endometriosis is related to the over secretion of cytokines such as prostaglandins and metalloproteinases, which damages the functions of ovary, peritoneum, fallopian tube and endometrium, and leads to the limitation of follicle generation, fertilization and implantation (28). The widespread adhesion in the pelvic cavity will hinder the release of oocytes, prevent sperm from entering the abdominal cavity or inhibit the oviduct from collecting eggs, thus reducing fertility. The reason of infertility caused by moderate and severe endometriosis may be that the disease causes premature loss of ovarian follicle reserve, abnormal follicular development, and decreased fertilization potential of oocytes. While phloroglucinol directly acts on the smooth muscle of gastrointestinal tract and genitourinary tract (29). It is a myophilic, non-atropine, non-papaverine, pure smooth muscle spasmolytic drug, which can effectively reduce the abnormal contraction of uterine smooth muscle and maintain the normal function of fallopian tube and endometrium. Phloroglucinol, as a safe, effective, economical, and practical smooth muscle antispasmodic, could significantly inhibit abnormal uterine smooth muscle contraction in young patients and reduce excessive and frequent endometrial peristalsis, thereby improving clinical pregnancy outcomes. As such, this approach is anticipated to become a new technique for improving pregnancy outcomes among infertile patients with endometriosis. The present research was a retrospective study comprising a small sample size, however, and its results will require confirmation by multicenter clinical studies with larger sample sizes.