Embryology · General embryology: weeks 1 to 8

Trophoblast and hCG; gestational trophoblastic disease !

Embryonic structure

The trophoblast is the outer layer of the blastocyst that invades the uterus, builds the placenta and makes the pregnancy hormone.

Updated 2026-11-01

See it in 3D: Body of uterus · Ovaries

What it is

The trophoblast is the outer cell layer of the blastocyst. It splits into the cytotrophoblast, which proliferates, and the syncytiotrophoblast, which invades the endometrium and makes human chorionic gonadotropin (hCG). hCG keeps the corpus luteum going early in pregnancy, and measuring it diagnoses and follows pregnancy and gestational trophoblastic disease (mole, choriocarcinoma).

Key facts

hCG appears in blood about 10 days after conception and in urine soon after.
In normal early pregnancy hCG rises fast (doubling in about two days) and peaks around 8–10 weeks' gestation.
The α subunit of hCG is shared with LH, FSH and TSH; very high levels can stimulate the thyroid.
Extravillous cytotrophoblast remodels the maternal spiral arteries; shallow invasion is linked to preeclampsia.
High second-trimester hCG is part of Down syndrome screening.

Structure

Cytotrophoblast: an inner layer of single dividing cells that feed the syncytium.
Syncytiotrophoblast: a multinucleated mass with no cell borders, in contact with maternal blood.
Chorionic villi: primary (trophoblast only), secondary (with mesoderm) and tertiary (with vessels).

What it becomes

Chorion and the fetal part of the placenta.
Maternal–fetal barrier and exchange in the villi.
Hormone production: hCG, progesterone, estrogens and placental lactogen.

Complete and partial mole

Feature	Complete	Partial
Karyotype	46,XX (or XY), all paternal	69,XXY or XXX (triploid)
Origin	Empty egg + one sperm that duplicates (or two)	Normal egg + two sperm
Fetal tissue	None	Fetal parts present
hCG	Very high	High, less marked
p57	Negative	Positive
Risk of persistent neoplasia	Higher	Lower

Markers

β-hCG
p57 (KIP2)
46,XX androgenetic
69,XXY

Clinical notes

Mole: first-trimester bleeding, uterus large for dates, hyperemesis, early preeclampsia and signs of hyperthyroidism.
Ultrasound of a complete mole: a 'snowstorm' or 'bunch of grapes' pattern.
Choriocarcinoma: a trophoblastic tumor that spreads early (lung) and responds very well to chemotherapy.
hCG that rises slowly or plateaus: suspect ectopic or nonviable pregnancy.
After a mole is evacuated, hCG is followed until normal to detect persistent neoplasia.

Test yourself

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

Which cells make hCG, and what is its main job in early pregnancy?

The syncytiotrophoblast. hCG maintains the corpus luteum, which keeps making progesterone until the placenta takes over.

What is the typical karyotype of a complete mole, and where do its chromosomes come from?

46,XX, all paternal: an egg without a nucleus fertilized by one sperm that duplicates (or by two).

Why can a molar pregnancy cause hyperthyroidism?

hCG shares its α subunit with TSH and, at very high levels, stimulates the TSH receptor.

What does a slowly rising hCG on serial early tests suggest?

An ectopic or nonviable intrauterine pregnancy; it is read together with ultrasound.

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See it in 3D

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

← All of 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: Langman's Medical Embryology, 13th ed., ch. 4: Second Week of Development: Bilaminar Germ Disc (Wolters Kluwer, 2015); HCG blood test - quantitative: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Hydatidiform mole: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.