Embryology · General embryology: weeks 1 to 8
Week 1: cleavage, morula, blastocyst and implantation !
- Stage
In week one the zygote divides on its way down the tube, reaches the uterus as a blastocyst and starts to implant.
Updated 2026-11-01
See it in 3D: Body of uterus · Endometrium · Uterine tubes · Ampulla of uterine tube
What it is
In the first week after fertilization the zygote divides into ever smaller cells (blastomeres) without growing, still inside the zona pellucida. Around day 3–4 the morula enters the uterine cavity, forms a cavity and becomes a blastocyst, with an inner cell mass (the future embryo) and an outer trophoblast (the future placenta). Around day 6 the blastocyst leaves the zona pellucida and attaches to the endometrium, usually on the posterior wall of the uterine body.
Key facts
- Day 1–2: first divisions (two cells by about 30 hours).
- Day 3: a morula of 12–16 cells; compaction happens around the 8-cell stage.
- Day 4–5: blastocyst with embryoblast (inner cell mass) and trophoblast.
- Day 5: hatching, the blastocyst leaves the zona pellucida.
- Day 6–7: implantation begins in a secretory-phase (progesterone) endometrium.
- Before week 3 most insults follow the all-or-none rule.
What happens, in order
- Zygote in the ampulla of the tube.
- Cleavage into blastomeres as it moves along the tube.
- Compaction: cells bind tightly and sort into inner and outer cells.
- Morula: enters the uterine cavity around day 3–4.
- Blastocyst: a cavity (blastocoele) forms; embryoblast at one pole, trophoblast around it.
- Hatching from the zona pellucida.
- Trophoblast at the embryonic pole attaches to the endometrial epithelium and begins invading.
Clinical notes
- Ectopic pregnancy: implantation outside the uterus, most often in the tubal ampulla; risks: tubal damage, pelvic infection, prior ectopic.
- Placenta previa: low implantation, near the internal cervical os.
- Monozygotic twins: splitting at the two-cell or morula stage gives twins with two chorions and two amnions; splitting the inner cell mass gives a shared chorion.
- Many embryos with chromosomal anomalies are lost this week before the pregnancy is noticed.
Test yourself
Think of the answer, then tap the question to see it.
Which part of the blastocyst forms the embryo, and which forms the placenta?
The inner cell mass (embryoblast) forms the embryo; the trophoblast forms the fetal part of the placenta.
When and where does implantation usually begin?
Around day 6 after fertilization, in the endometrium of the posterior wall of the uterine body.
What is the commonest site of ectopic pregnancy?
The uterine tube, especially the ampulla.
What does the all-or-none rule of the first two weeks mean?
An insult usually either kills the embryo or lets it recover without defects, because its cells can still replace one another.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Fertilization: capacitation, acrosome reaction and zona reaction
- Week 2: the bilaminar disc ('the week of twos')
- Trophoblast and hCG; gestational trophoblastic disease
- Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion
- Abnormal placentation: previa and the accreta spectrum
- Gestational age vs fertilization age: how pregnancy is dated
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. 3: First Week of Development: Ovulation to Implantation (Wolters Kluwer, 2015); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Ectopic pregnancy: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.