Embryology · General embryology: weeks 1 to 8
Week 2: the bilaminar disc ('the week of twos') !
- Stage
In week two the embryo finishes implanting and organizes into two layers, with two cavities around it.
Updated 2026-11-01
See it in 3D: Body of uterus · Endometrium
What it is
In week two everything comes in pairs: the trophoblast splits into cytotrophoblast and syncytiotrophoblast, the embryoblast into epiblast and hypoblast (the bilaminar disc), two cavities appear (amniotic cavity and yolk sac) and the extraembryonic mesoderm splits into two layers. The syncytiotrophoblast invades the endometrium, taps maternal sinusoids and begins making hCG, the basis of pregnancy tests.
Key facts
- Day 8: the syncytiotrophoblast invades the endometrium; the amniotic cavity appears within the epiblast.
- Day 9: lacunae in the syncytiotrophoblast; hypoblast lines the primitive yolk sac.
- Days 11–12: lacunae connect with maternal sinusoids: the uteroplacental circulation begins.
- Day 13: primary villi; definitive yolk sac; the connecting stalk joins the disc to the trophoblast.
- The epiblast will form all three germ layers; the hypoblast is displaced and forms no embryonic tissue.
- Around day 13, light implantation bleeding can be mistaken for a period and throw off the calculated date.
What happens, in order
- The blastocyst sinks fully into the endometrium; a fibrin plug closes the entry site.
- Trophoblast → cytotrophoblast (inner, dividing) and syncytiotrophoblast (outer, invasive).
- Embryoblast → epiblast (tall, dorsal) and hypoblast (cuboidal, ventral): the bilaminar disc.
- The amniotic cavity (above the epiblast) and the yolk sac (below the hypoblast) form.
- Extraembryonic mesoderm appears and splits into somatic and splanchnic layers, leaving the chorionic cavity.
- Cytotrophoblast pushes into the syncytium: primary villi.
- The prechordal plate, a thickening of the hypoblast, marks the cranial end.
Clinical notes
- hCG from the syncytiotrophoblast is detectable in blood by the end of this week, about 10 days after conception.
- Hydatidiform mole: trophoblast proliferates without a normal embryo; hCG is very high.
- Ectopic pregnancy: trophoblast invading the tube can rupture it and bleed in the first trimester.
- Week 2: the all-or-none rule still applies to most exposures.
Test yourself
Think of the answer, then tap the question to see it.
Which two layers make up the bilaminar disc?
The epiblast (dorsal, facing the amniotic cavity) and the hypoblast (ventral, facing the yolk sac).
Which trophoblast layer makes hCG?
The syncytiotrophoblast.
When does the uteroplacental circulation begin?
Around days 11–12, when syncytiotrophoblast lacunae connect with maternal sinusoids.
Why can bleeding around day 13 throw off the dating of a pregnancy?
Implantation bleeding falls near the expected period and can be mistaken for one.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Week 1: cleavage, morula, blastocyst and implantation
- Trophoblast and hCG; gestational trophoblastic disease
- Week 3: gastrulation and the trilaminar disc
- Fetal membranes: amnion, chorion, yolk sac and allantois
- Placenta: structure, circulation and functions
Related conditions
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); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.