Miscarriage is the spontaneous loss of an intrauterine pregnancy before 24 weeks of gestation and affects 1 in 5 clinical pregnancies (50). Miscarriage predominantly occurs during the first trimester of pregnancy and the majority of early miscarriages are due to a genetic abnormality of the developing embryo, however other causes include endocrine, anatomical, and immunological factors (51).

Miscarriage diagnosis can be challenging as often a pregnancy is failing for a time before pregnancy loss has conclusively been confirmed. This uncertainty can exacerbate the psychological burden related to investigating possible miscarriage, with up to 6% of women suffering from moderate-severe depression, 17% from moderate-severe anxiety and 18% from post-traumatic stress disorder (52). To date, there is no clinical predictor of miscarriage, however recent data demonstrates a potential for kisspeptin as a biomarker of miscarriage.

Kisspeptin levels (adjusted for gestation) are markedly reduced by 60-79% in women with miscarriage compared to healthy pregnancy (20–25) ( Table 2.1 ). Above average levels, when corrected for gestational age, are reassuring with a <1% chance of miscarriage (20), whereas kisspeptin levels 95% lower than the median for that gestation are associated with up to an 85% chance of miscarriage. Concordantly, KISS-1 expression is decreased in the placentae of women with recurrent spontaneous abortion compared to those who undergo voluntary termination of pregnancy (11). Furthermore, whilst kisspeptin’s high diagnostic performance for identifying miscarriage is maintained in late-first trimester pregnancies (>8 weeks of gestation), that of β-human chorionic gonadotrophin (β-hCG) worsens (20). Thus, the combination of both kisspeptin and β-hCG can be used to ensure high diagnostic accuracy at all gestations (AUCROC 0.92, 95% CI 0.89-0.95) (20, 22, 24). Kisspeptin has also been shown to reflect different types of miscarriage, with lower levels reported in complete (no retained products of conception) versus incomplete (retained products of conception) or missed (empty gestational sac or a foetal pole with no heartbeat) miscarriage (20). Additionally, both kisspeptin and β-hCG levels decline with closer proximity to miscarriage confirmation, and therefore repeat measurements every 1-2 weeks could enable further risk-stratification of miscarriage risk in clinical practice (20).

Studies involving women with infertility who undergo assisted reproductive techniques (in vitro fertilisation, intracytoplasmic sperm insemination (ICSI) or frozen thawed embryo transfer) have found reduced β-hCG levels in miscarriage compared to controls, but no difference in kisspeptin levels (53, 55). These findings may be due to the very early gestations at which kisspeptin levels were assessed (2-3 weeks following, or even before, pregnancy confirmation) (53, 55). Indeed, kisspeptin may not be expressed in the placenta at high levels prior to 6 weeks of gestation, suggesting that β-hCG levels may be more useful at these very early gestations (26).

Ectopic pregnancy (EP) affects 2% of pregnancies and occurs when a fertilised ovum implants and develops outside the uterine cavity, most commonly within the fallopian tube (56). EP can result in tubal rupture and accounts for 9-13% of all pregnancy-related deaths in developed countries and can compromise a woman’s future fertility (57). EP is currently diagnosed by serial β-hCG measurements in combination with ultrasound, although laparoscopy is often required to provide a definitive diagnosis (58). The sensitivity and specificity of these tests significantly decrease in the case of pregnancies of unknown location (PUL) as false positive or negative diagnoses may occur. This is important as an incorrect diagnosis may lead to termination of a healthy pregnancy (59). Accordingly, different biomarkers have been investigated in an attempt to improve the diagnostic accuracy of EP, including kisspeptin.

Some studies have found that kisspeptin levels in EP are lower than in healthy pregnancy but higher than in miscarriage (25, 26). However, another study demonstrated that kisspeptin levels are not significantly altered between women with viable intrauterine pregnancies (VIUPs) and those with either EP or failing or persistent PUL, after adjusting for confounding variables (20) ( Table 2.2 ). Current evidence remains limited, and larger studies are required to determine kisspeptin’s performance as a diagnostic marker in EP at early gestations (<6 weeks).

Hypertensive disorders affect 5% of all pregnancies (60) and include pre-existing chronic hypertension (CH), pregnancy induced hypertension (PIH) and pre-eclampsia (PE). PIH is defined as new onset hypertension (BP ≥140/90mmHg) occurring after 20 weeks of gestation, PE is PIH with proteinuria (urine >3g/24 hours) or significant end-organ dysfunction, and severe PE is the presence of at least one of: hypertension (BP≥160/110 mmHg), visual disturbance, chest pain, dyspnoea, pulmonary oedema, seizures, or neonatal distress (61). PE is further classified, according to the onset of clinical features, into early-onset PE (EPE <34 weeks of gestation) and late-onset PE (LPE ≥34 weeks of gestation). EPE is associated with impaired trophoblast invasion, defective spiral artery remodelling and adverse perinatal complications including IUGR (62). LPE occurs due to hypoxic stress and impaired perfusion but is less likely to compromise foetal growth (63, 64). Currently, PE diagnosis is based on early pregnancy risk factor screening, uterine artery Doppler velocimetry and biomarkers such as PPAP-A or placental growth factor (PlGF) (61). Kisspeptin has been implicated in the pathogenesis of PE through reduced angiogenesis, decreased cytotrophoblast invasion and increased trophoblast apoptosis, and thus could have potential in predicting PE (16–18).

Levels of circulating kisspeptin in HDP vary in the literature, and largely differ according to HDP subtype, severity, and onset ( Table 2.3 ). Most of the studies report reduced circulating kisspeptin levels in PE compared to normotensive pregnant controls (29, 32, 40–43, 65) and therefore kisspeptin is considered to reflect placental dysfunction. However, expression of KISS-1, which inhibits trophoblast invasion and results in defective transformation of the spiral arteries, is increased in the placentae of PE pregnancies, thus supporting its role in the pathophysiology of PE (32–34, 36, 66) ( Table 1 ). Nonetheless, there are also some reports of decreased KISS-1 expression in PE placentae (37, 38) ( Table 1 ). Furthermore, evidence suggests that circulating kisspeptin levels decline as the severity of PE increases, which could also reflect reduced placental mass in more severe disease. Indeed, both circulating kisspeptin levels and placental mass is reduced in EPE compared to LPE (28, 42, 67). Additionally, pregnant women with pre-existing hypertension and PE, states associated with a higher burden of disease, have reduced kisspeptin levels compared to PIH (40).

Whilst most studies demonstrate reduced kisspeptin levels in PE, a recent study found that kisspeptin levels are increased in HDP during the third trimester of pregnancy ( Table 2.3 ). However, there was no association between circulating kisspeptin levels and severity of PET (28). It is likely that complexity in the categorisation, severity, and onset of PET, and the need for correction for possible confounders such as BMI and gestational age, could explain differences between kisspeptin levels observed in the current studies. Larger observational studies that are carefully designed to address these and look at each PET-subset throughout pregnancy would therefore be valuable in resolving these inconsistencies.

During pregnancy a physiological rise in maternal insulin resistance provides glucose to the developing foetus (68, 69). This insulin resistance leads to maternal pancreatic β-cell adaptation and increased insulin secretion. Failure of these changes results in gestational diabetes mellitus (GDM), which affects up to 20% of pregnancies worldwide (70).

Kisspeptin receptors are expressed in pancreatic β-cells (71) and have been implicated in β-cell adaptation during pregnancy. Exogenous kisspeptin administration has variable physiological effects on the glucose-dependent regulation of pancreatic beta-cells. For instance, KISS-1 peptide (KP-145) (71), KP-13 (72), KP-10 (72–74) potentiates glucose-stimulated insulin secretion (GSIS) in animal and human islets in-vitro. KP-54 increases GSIS in healthy men following an intravenous glucose tolerance test (IVGTT), which induces high glucose levels (75). On the other hand, Vikam and colleagues have found that KP-13 and KP-54 drives dose-dependent inhibitory effects on insulin secretion in mouse islets in the presence of lower glucose concentrations (2.8-11.1 mmol/l), compared to controls, which is not observed at higher glucose concentrations (76). Furthermore, chronic administration of KP-10 in non-pregnant mice enhances GSIS and improves glucose tolerance (47). Interestingly, hyperlipidaemia, impaired glucose tolerance (IGT) and weight gain develops in Kiss-1r-null female mice exclusively, thus suggesting sexual dimorphism in kisspeptin’s effects on metabolism and glucose homeostasis (77).

In late gestation murine pregnancy, β-cell specific Kiss-1r-knockout models and pharmacological inhibition of Kiss-1r leads to reduced GSIS and development of IGT, which is not observed in non-pregnant states or wild-type controls (47). This supports a role for β-cell kisspeptin signalling in the regulation of glucose homeostasis during pregnancy. Loss of kisspeptin signalling in the ß-cell-specific Kiss-1r-knockout models also attenuates the increased ß-cell proliferation normally seen during murine pregnancy when assessed with bromodeoxyuridine (BrdU) labelling. Nonetheless, the levels are not reduced to non-pregnant levels, suggesting contribution of other signals in pancreatic β-cell proliferation during pregnancy (47, 78).

In human pregnancies with GDM, placental KISS-1 and KISS-1R expression is elevated in the third trimester (35, 45) ( Table 1 ), whereas circulating kisspeptin levels have been either lower (40, 47) or not significantly altered (28, 46) ( Table 2.4 ). Finally, Bowe and colleagues have demonstrated a positive correlation between third trimester kisspeptin levels and oral glucose–stimulated insulin levels at 60 minutes (r^{2 =} 0.18; P < 0.0001) and AUC serum insulin over the OGTT (r² = 0.13; P=0.0013) in women with GDM (47).

Pre-term birth (PTB) is defined as delivery prior to 37 weeks of gestation and affects 11% of pregnancies (79, 80). Kisspeptin has been proposed to initiate labour through increased oxytocin neuronal firing rate in pregnant rats and thus may play a potential role in PTB (81). Gestation adjusted kisspeptin levels are higher in PTB-affected pregnancies than in control pregnancies during the late-first trimester, with the adjusted odds of PTB being increased by 20% (95% CI, 1-42%) for every 1 nmol/L increase in plasma kisspeptin (28) ( Table 2.5 ). Furthermore, KISS-1 mRNA expression is higher in preterm placentae than in term placentae delivered vaginally or by Caesarean section thus indicating that increased kisspeptin expression could be involved in the induction of labour (27) ( Table 1 )W. However, no alteration in circulating kisspeptin levels have been reported to date during the third trimester between healthy pregnancy and PTB and thus more data is needed to elucidate whether there are changes in kisspeptin levels preceding and around the time of spontaneous labour (27, 28).

Foetal growth restriction (FGR) encompasses both intrauterine growth restriction (IUGR, foetal weight <10^th centile for gestational age with abnormal umbilical artery doppler results) and small for gestation age (SGA, delivery weight <10^th percentile for gestational age) (82, 83). FGR is thought to arise from abnormal trophoblast invasion and spiral artery remodelling that limits oxygen supply to the placenta (84, 85). The resulting ischemic injury generates reactive oxygen species which lead to apoptosis and restriction of placental and foetal growth (84, 85). To date, four studies have demonstrated significantly reduced kisspeptin levels in FGR versus healthy pregnancy in all three trimesters (28–31) ( Table 2.6 ). Thus, low circulating kisspeptin levels could reflect low placental mass in pregnancies affected by FGR.

Gestational trophoblastic disease (GTD) is characterised by an abnormal proliferation of placental tissue and comprises of choriocarcinoma, invasive mole, placental site trophoblastic tumour and epithelioid trophoblastic tumour (86). Molar pregnancy is a benign form of GTD, whereas choriocarcinomas are more aggressive, however both exhibit high β-hCG levels and respond well to chemotherapy (87). Serum β-hCG measurement aids with GTD diagnosis, staging and prognostication before and after chemotherapy (88).

KISS-1 and KISS-1R expression is significantly lower in malignant choriocarcinoma cells compared to molar and healthy pregnancies (38, 48) ( Table 1 ). Conversely, circulating kisspeptin levels are elevated in malignant GTD compared to healthy pregnancies but significantly decline following chemotherapy (7) ( Table 2.7 ). The increased circulating kisspeptin levels could reflect an increased malignant trophoblast mass rather than an elevation in cellular KISS-1 expression (89). Thus, kisspeptin levels can be altered in choriocarcinomas and other GTDs, which is interesting when considering the original identification of KISS-1 as an anti-metastatic gene.