Physiology · Reproduction, pregnancy and the fetus
The placenta and pregnancy hormones !
- Concept
- Exam priority: medium
The placenta passes oxygen and food to the baby and works as a hormone gland that keeps the pregnancy going.
Updated 2026-11-01
Also called: hCG, Luteal–placental shift
See it in 3D: Ovaries · Body of uterus
What it is
The placenta is the exchange organ between mother and fetus and a large endocrine gland. Its syncytiotrophoblast makes hCG, which keeps the corpus luteum alive in the first weeks, then takes over progesterone and estrogen production itself. Other placental hormones redirect maternal metabolism toward the fetus.
Key facts
- Exchange happens across the villous membrane: oxygen, CO₂ and lipid-soluble molecules diffuse; glucose uses facilitated transport (GLUT1); amino acids and calcium are actively transported.
- IgG crosses the placenta by FcRn-mediated transcytosis (passive immunity, and the route of anti-Rh antibodies); IgM does not.
- Maternal and fetal blood do not mix; the fetus extracts oxygen thanks to HbF, a higher hemoglobin concentration and the double Bohr effect.
- The placenta weighs about 500 g at term and receives about 500–700 mL/min of maternal blood.
- hCG shares an α subunit with LH, FSH and TSH; very high levels can stimulate the thyroid (gestational thyrotoxicosis, hyperemesis).
Typical values
| Parameter | Value | Note |
|---|---|---|
| hCG first detectable (serum) | ≈8–10 days after ovulation | |
| hCG doubling time early on | ≈48 h | a slower rise suggests a failing or ectopic pregnancy |
| hCG peak | ≈8–10 weeks | then falls to a plateau |
| Luteal–placental shift | ≈7–9 weeks | |
| Placental weight at term | ≈500 g |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Placental and pregnancy hormones
| Hormone | Made by | Main job |
|---|---|---|
| hCG | Syncytiotrophoblast | Maintains the corpus luteum; basis of pregnancy tests |
| Progesterone | Corpus luteum, then placenta | Keeps the decidua and a quiet myometrium |
| Estriol and estradiol | Placenta (from fetal DHEA-S) | Uterine growth and blood flow, breast ducts |
| Human placental lactogen | Syncytiotrophoblast | Maternal insulin resistance, fetal fuel supply |
| Placental CRH | Placenta | Rises toward term; linked to timing of labor |
| Relaxin | Corpus luteum, placenta | Vasodilation, softening of the cervix and pelvic ligaments |
Clinical notes
- Ectopic pregnancy: hCG rises too slowly and no intrauterine sac is seen above the discriminatory level.
- Molar pregnancy: abnormal trophoblast makes very high hCG; it can cause hyperemesis, early preeclampsia and hyperthyroidism.
- Abnormal remodeling of maternal spiral arteries by the trophoblast underlies preeclampsia and fetal growth restriction.
Test yourself
Think of the answer, then tap the question to see it.
Why does removing the corpus luteum at 5 weeks end a pregnancy but not at 12 weeks?
Before about 7–9 weeks progesterone comes from the corpus luteum (kept alive by hCG). After the luteal–placental shift the placenta makes enough progesterone on its own.
How does human placental lactogen help the fetus?
It raises maternal insulin resistance and lipolysis, so the mother burns more fat and leaves more glucose and amino acids to cross to the fetus.
Why can very high hCG cause hyperthyroidism?
hCG shares the α subunit with TSH and its β subunit resembles TSH's, so at very high levels (molar pregnancy, twins) it weakly activates the TSH receptor.
Why can maternal anti-D antibodies harm the fetus while anti-A IgM antibodies usually do not?
The placenta transports IgG (via FcRn) but not IgM. Anti-D is usually IgG; natural anti-A and anti-B are mostly IgM.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Maternal physiological changes in pregnancy
- Fertilization and implantation
- Parturition: onset and uterine contraction
- The menstrual cycle: ovary and endometrium
- Rh incompatibility
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 17.8 Gonadal and Placental Hormones (OpenStax (Rice University), 2022); Physiology, Placenta (StatPearls (NCBI Bookshelf), 2023); Hydatidiform Mole (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 83). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.