Peptide Hormones of the Placenta
Peptides in Pregnancy
7,000× higher kisspeptin
Circulating kisspeptin levels increase up to 7,000-fold during pregnancy due to placental production, making it a potential biomarker for placental health.
Hu et al., European Journal of Obstetrics & Gynecology, 2019
Hu et al., European Journal of Obstetrics & Gynecology, 2019
View as imageThe placenta is a temporary endocrine organ that exists for roughly 40 weeks and produces more peptide hormones than most permanent glands. It synthesizes human chorionic gonadotropin (hCG), kisspeptin, corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), growth hormone-releasing hormone (GHRH), natriuretic peptides, and placental variants of growth hormone and lactogen. These peptides regulate everything from the initial maintenance of pregnancy to the timing of labor. Ahmadi et al. (2025) reviewed the full scope of placental peptide hormone production and the cellular trafficking mechanisms that control their secretion.[1] For a broader look at peptide biology during pregnancy, see our pillar article on oxytocin and breastfeeding.
Key Takeaways
- The placenta produces at least seven classes of peptide hormones that regulate maternal physiology, fetal development, and the timing of parturition (Ahmadi et al., 2025)
- Plasma kisspeptin increases up to 7,000-fold during pregnancy, and altered levels may predict miscarriage and preeclampsia (Hu et al., 2019)
- VIP promotes cytotrophoblast invasion and metabolic function, and its deficiency is associated with preeclampsia and fetal growth restriction (Merech et al., 2025)
- The GHRH/GHRH receptor axis regulates trophoblast cell viability and apoptosis, linking placental growth factor signaling to pregnancy outcomes (Liu et al., 2016)
- Natriuretic peptides maintain fetal cardiovascular homeostasis, with their metabolism differing across the fetoplacental circulation (Miyoshi et al., 2019)
- Placental CRH rises exponentially in the third trimester and is thought to function as a "placental clock" that helps determine the timing of labor
hCG: The Hormone That Sustains Early Pregnancy
Human chorionic gonadotropin is the most clinically familiar placental peptide. Produced by syncytiotrophoblast cells within days of implantation, hCG is the molecule detected by pregnancy tests. But its biological role extends far beyond signaling pregnancy status.
hCG maintains the corpus luteum during the first trimester, ensuring continued progesterone production until the placenta takes over steroid synthesis. Ahmadi et al. (2025) described how hCG is secreted through the constitutive secretory pathway, meaning it is released continuously without requiring a specific stimulus.[1] Production peaks around weeks 8-10 of gestation and then declines.
Beyond corpus luteum maintenance, hCG promotes angiogenesis in the uterine vasculature, modulates maternal immune tolerance of the fetus, and influences trophoblast differentiation. The placenta produces several hCG variants (hyperglycosylated hCG, free beta subunit) that have distinct biological activities and may serve as biomarkers for pregnancy complications including ectopic pregnancy, gestational trophoblastic disease, and chromosomal abnormalities.
Kisspeptin: The Placental Biomarker
Outside of pregnancy, kisspeptin is primarily known for its role in the hypothalamic control of GnRH secretion and puberty onset. During pregnancy, the placenta becomes a massive kisspeptin factory. Hu et al. (2019) reviewed kisspeptin's potential as a biomarker throughout pregnancy.[2]
Plasma kisspeptin levels increase dramatically during healthy pregnancy, rising up to 7,000-fold above non-pregnant levels. This increase is driven by placental syncytiotrophoblast production. The review found that kisspeptin levels could serve as biomarkers for several pregnancy complications:
- Miscarriage: Women who subsequently miscarried had lower first-trimester kisspeptin levels than those with viable pregnancies
- Preeclampsia: Altered kisspeptin levels in the first trimester may predict later preeclampsia development
- Gestational trophoblastic neoplasia: Kisspeptin levels correlate with the degree of trophoblast proliferation
- Ectopic pregnancy: Lower kisspeptin levels may help distinguish ectopic from intrauterine pregnancies
Kisspeptin also has functional roles in the placenta beyond being a circulating marker. It regulates trophoblast invasion into the maternal uterine wall during placentation, a process critical for establishing adequate blood supply to the developing fetus. Deficient trophoblast invasion is a key pathological feature of preeclampsia. For more on kisspeptin's reproductive roles, see our article on kisspeptin and ovulation.
VIP: Metabolic Rewiring of Trophoblasts
Vasoactive intestinal peptide has a newly characterized role in placental biology. Merech et al. (2025) demonstrated that VIP induces metabolic rewiring of human cytotrophoblast cells to promote their invasive phenotype.[3]
Extravillous cytotrophoblast (EVT) cells must invade the maternal uterine wall and remodel spiral arteries to establish the placental blood supply. This process requires specific metabolic adaptations. Merech et al. showed that VIP promotes the metabolic changes needed for EVT invasion, shifting cellular energy production to support the invasive phenotype.
Deficient EVT function underlies two major pregnancy complications: preeclampsia and fetal growth restriction (FGR). The finding that VIP drives the metabolic program needed for proper trophoblast invasion suggests that VIP signaling deficits could contribute to these conditions. This connects placental VIP biology to the broader story of how neuropeptides regulate tissue invasion and remodeling.
GHRH: Growth Regulation at the Placental Level
Growth hormone-releasing hormone is produced by the placenta and acts through local GHRH receptors on trophoblast cells. Liu et al. (2016) investigated the GHRH/GHRH receptor axis in placental choriocarcinoma cells and normal placental tissue.[4]
They found that the GHRH receptor regulates trophoblast cell viability and apoptosis. Downregulation of GHRH-R expression increased apoptosis, while GHRH stimulation promoted cell survival. The study also identified downstream signaling pathways (including MAPK and PI3K/AKT) through which GHRH influences trophoblast function.
This local GHRH system operates independently of the hypothalamic-pituitary GHRH axis. The placenta effectively creates its own growth-regulatory peptide environment, fine-tuning trophoblast proliferation and survival without requiring signals from the maternal brain. Similar autocrine/paracrine peptide systems have been identified in other placental cell types, supporting the concept of the placenta as a self-regulating endocrine organ.
Placental CRH: The Birth Clock
One of the most fascinating placental peptides is corticotropin-releasing hormone. While CRH is produced in the hypothalamus to regulate the stress response through ACTH and cortisol, the placenta produces CRH in quantities that dwarf hypothalamic output.
Placental CRH production increases exponentially during the third trimester. Unlike hypothalamic CRH, which is suppressed by cortisol through negative feedback, placental CRH is stimulated by cortisol, creating a positive feedback loop that accelerates toward parturition. This has led to the "placental clock" hypothesis: that placental CRH helps determine the timing of labor.
Women who deliver preterm have elevated CRH levels earlier in pregnancy, while women who deliver post-term have lower CRH trajectories. Maternal plasma CRH measured in the second trimester has been investigated as a predictor of preterm birth, though its clinical utility as a standalone biomarker remains limited by significant individual variation.
Placental CRH also influences fetal organ maturation, particularly lung surfactant production, and may help coordinate the fetal stress response during delivery. The connection between the placental CRH system and the HPA stress axis illustrates how the placenta mirrors and extends maternal neuroendocrine systems.
Natriuretic Peptides in the Fetoplacental Circulation
Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) play critical roles in maintaining cardiovascular homeostasis during pregnancy. Miyoshi et al. (2019) studied the metabolism of these peptides in the fetoplacental circulation of fetuses with congenital heart defects.[5]
They measured natriuretic peptide concentrations in maternal vein, umbilical artery, and umbilical vein samples, revealing that the placenta actively metabolizes these peptides rather than simply allowing them to pass through. The fetoplacental circulation has its own natriuretic peptide regulatory system, distinct from the maternal circulation.
Quek et al. (2025) reviewed the clinical utility of BNP levels in pregnancy more broadly.[6] BNP levels help identify pregnant women with cardiac complications, and placental production of natriuretic peptides contributes to the maternal cardiovascular adaptations necessary for pregnancy, including increased blood volume and reduced systemic vascular resistance.
Oxytocin: The Link Between Placenta and Labor
While oxytocin is primarily produced in the hypothalamus, the placenta plays a critical role in oxytocin signaling during labor. Uvnas-Moberg et al. (2024) reviewed the physiology and pharmacology of oxytocin in labor and the peripartum period.[7]
The placenta expresses oxytocin receptors, and oxytocin receptor density in the uterus increases dramatically toward term. The placenta also produces enzymes that metabolize oxytocin, helping to regulate the balance between oxytocin stimulation and clearance. Hermesch et al. (2024) reviewed the clinical applications of oxytocin in labor management, including induction and augmentation protocols.[8]
The interplay between placental CRH (which increases toward term), rising oxytocin receptor expression, and prostaglandin production creates the endocrine conditions for labor onset. No single peptide triggers labor; rather, it is the convergence of multiple placental and maternal peptide signals reaching a tipping point.
Human Placental Lactogen and Placental Growth Hormone
Two additional peptide hormones produced by the placenta deserve mention for their metabolic effects on the mother.
Human placental lactogen (hPL) is secreted in large quantities by the syncytiotrophoblast, reaching peak levels in the third trimester. It acts as a metabolic hormone in the mother, promoting lipolysis (fat breakdown) and insulin resistance to ensure that glucose is preferentially available to the fetus. hPL is structurally related to growth hormone and prolactin, and it also helps prepare the mammary glands for lactation.
Placental growth hormone (hPGH) is a variant of pituitary growth hormone produced exclusively by the placenta. It gradually replaces maternal pituitary growth hormone in the circulation during the second and third trimesters. Ahmadi et al. (2025) noted that hPGH plays a key role in maternal metabolic adaptation, stimulating IGF-1 production and promoting nutrient mobilization for fetal growth.[1]
The combination of hPL and hPGH creates the progressive insulin resistance of pregnancy, which is physiologically normal but can become pathological in gestational diabetes. Understanding these placental peptides helps explain why pregnancy is a metabolic stress test that reveals latent susceptibility to diabetes and cardiovascular disease.
How Placental Peptides Are Secreted
The placenta uses distinct secretory pathways for different peptide hormones. Ahmadi et al. (2025) described two primary routes: the constitutive secretory pathway (continuous release, used by hCG and hPL) and the regulated secretory pathway (stimulus-dependent release, used for neuropeptide-type hormones).[1]
The syncytiotrophoblast, which forms the outer layer of the placenta in direct contact with maternal blood, is the primary secretory cell type. This multinucleated cell layer has a unique structure: it is a continuous syncytium without individual cell boundaries, formed by fusion of underlying cytotrophoblast cells. This structure gives it an enormous surface area for hormone secretion directly into the maternal circulation.
The regulated secretory pathway allows the placenta to modulate hormone release in response to local signals, including oxygen tension, glucose levels, and inflammatory mediators. This responsiveness is what makes placental peptide levels potential biomarkers for pregnancy health: when the placenta is stressed or dysfunctional, its peptide hormone output changes in measurable ways.
Why Placental Peptide Biology Matters Beyond Pregnancy
Understanding placental peptide hormones has implications beyond obstetrics. The placenta provides a model for how a temporary organ establishes its own endocrine system, complete with autocrine, paracrine, and endocrine signaling loops. Many of the peptides produced by the placenta (CRH, kisspeptin, VIP, GHRH, natriuretic peptides) are also produced by tumors, leading to paraneoplastic syndromes that mimic aspects of pregnancy physiology.
Placental peptide biomarkers also represent one of the most practical near-term applications of peptide research. First-trimester kisspeptin or CRH measurements could identify high-risk pregnancies before complications develop, enabling earlier intervention. The relationship between GLP-1 drugs and pregnancy outcomes further illustrates why peptide biology in the placental environment demands careful study.
The Bottom Line
The placenta produces a diverse array of peptide hormones including hCG, kisspeptin, CRH, VIP, GHRH, and natriuretic peptides. Each serves specific functions in pregnancy maintenance, fetal development, and labor timing. Kisspeptin's 7,000-fold increase during pregnancy makes it a promising biomarker for complications. Placental CRH operates as a "birth clock" through a positive cortisol feedback loop. VIP drives the metabolic reprogramming needed for trophoblast invasion. These findings are expanding both clinical obstetrics and the fundamental understanding of how temporary organs build peptide signaling systems.