[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Fetal period, placenta and twins](https://durgical.com/en/embryology/#a-fetal) › Placenta: structure, circulation and functions

[Embryology](https://durgical.com/en/embryology/) · [Fetal period, placenta and twins](https://durgical.com/en/embryology/#a-fetal)

# Placenta: structure, circulation and functions

- Embryonic structure

The placenta links the fetus to the mother: it swaps oxygen and nutrients, clears waste and makes the hormones of pregnancy.

Updated 2026-11-01

**See it in 3D:** [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) · [Uterine artery](https://durgical.com/en/anatomy/uterine-artery/)

## Key points

- Fetal part: chorion frondosum (trophoblast plus extraembryonic mesoderm); maternal part: decidua basalis.
- The syncytiotrophoblast makes hCG, progesterone, estrogens (mainly estriol) and somatomammotropin (placental lactogen).
- hCG keeps the corpus luteum going for the first two months; then the placenta takes over progesterone production.
- IgG is the only immunoglobulin that crosses the placenta: passive immunity of the newborn.
- Preeclampsia starts with incomplete remodelling of the spiral arteries by cytotrophoblast.
- The placental membrane thins from month 4: syncytium lies directly on fetal endothelium.

## What it is

The placenta has a fetal part, the chorion frondosum, and a maternal part, the decidua basalis. Maternal blood bathes the chorionic villi in the intervillous space without mixing with fetal blood. Besides exchanging gases and nutrients, it makes the hormones that sustain pregnancy and transfers IgG antibodies; when it fails, the result is preeclampsia, growth restriction or abruption.

## Structure

- Villi: primary (trophoblast only), secondary (with mesoderm) and tertiary (with fetal vessels), the last by the end of week 3.
- Chorionic plate on the fetal side and decidual plate on the maternal side; between them, the intervillous space full of maternal blood.
- Decidual septa divide the maternal surface into 15 to 20 cotyledons.
- Some 80 to 100 maternal spiral arteries pour blood into the intervillous spaces; endometrial veins drain it back.
- At term it is about 15 to 25 centimetres across and about 3 centimetres thick, and weighs roughly half a kilogram.

## Products and functions of the placenta

| Product or function | What it does |
| --- | --- |
| hCG | Maintains the corpus luteum; basis of the pregnancy test |
| Progesterone | Supports the endometrium and the pregnancy |
| Estriol | Uterine and breast growth; needs the fetal liver and adrenal |
| [Somatomammotropin (hPL)](https://durgical.com/en/conditions/gestational-diabetes-mellitus/) | Maternal insulin resistance: more glucose for the fetus |
| IgG transport | Passive immunity of fetus and newborn |
| Gas and nutrient exchange | Oxygen, glucose and amino acids in; CO2 and urea out |

## Markers

- hCG
- hPL
- Estriol
- IgG
- Syncytiotrophoblast

## Clinical notes

- Preeclampsia: shallow placentation, placental ischemia and maternal hypertension from week 20.
- Placental abruption: separation before delivery with painful bleeding and a tense uterus.
- Placental insufficiency: growth restriction, oligohydramnios and fetal distress.
- The placenta is not a full barrier: many drugs, alcohol, viruses and Treponema pallidum cross it.

## Memory aid

The syncytium is the factory: it makes the pregnancy hormones and is also the only wall between the mother's blood and the baby's.

## Test yourself

Think of the answer, then tap the question to see it.

## Which tissue forms the maternal part of the placenta?

The decidua basalis, the endometrium beneath the implantation site.

## Which placental hormone maintains the corpus luteum in early pregnancy?

Human chorionic gonadotropin (hCG), made by the syncytiotrophoblast.

## Which maternal immunoglobulin crosses the placenta?

Only IgG, which gives the fetus and newborn passive immunity for the first months.

## Which placental failure lies at the root of preeclampsia?

Incomplete remodelling of the spiral arteries by extravillous cytotrophoblast, leaving a poorly perfused placenta.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/placenta-structure-circulation-and-functions/)

## See it in 3D

The structures of Durgical's 3D anatomy where it is found.

- [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/)
- [Uterine artery](https://durgical.com/en/anatomy/uterine-artery/)

## Related

- [Trophoblast and hCG; gestational trophoblastic disease](https://durgical.com/en/embryology/trophoblast-and-hcg-gestational-trophoblastic-disease/)
- [Fetal membranes: amnion, chorion, yolk sac and allantois](https://durgical.com/en/embryology/fetal-membranes-amnion-chorion-yolk-sac-and-allantois/)
- [Abnormal placentation: previa and the accreta spectrum](https://durgical.com/en/embryology/abnormal-placentation-previa-and-the-accreta-spectrum/)
- [Umbilical cord: two arteries, one vein](https://durgical.com/en/embryology/umbilical-cord-two-arteries-one-vein/)
- [Fetal growth and growth restriction](https://durgical.com/en/embryology/fetal-growth-and-growth-restriction/)
- [Principles of teratology: critical periods, dose and genotype](https://durgical.com/en/embryology/principles-of-teratology-critical-periods-dose-and-genotype/)

## Related conditions

- [Preeclampsia](https://durgical.com/en/conditions/preeclampsia/)
- [Placental abruption](https://durgical.com/en/conditions/placental-abruption/)
- [Eclampsia](https://durgical.com/en/conditions/eclampsia/)
- [Gestational diabetes mellitus](https://durgical.com/en/conditions/gestational-diabetes-mellitus/)

[← All of embryology](https://durgical.com/en/embryology/)

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: [Placental insufficiency: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001485.htm) (MedlinePlus, U.S. National Library of Medicine, 2024); [Placenta abruption - definition: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000901.htm) (MedlinePlus, U.S. National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/placenta-structure-circulation-and-functions/