Embryology · General embryology: weeks 1 to 8
Weeks 3–8: the embryonic period and organogenesis !
- Stage
Between weeks three and eight every organ forms; it is when the embryo is most vulnerable to teratogens.
Updated 2026-11-01
See it in 3D: Spinal cord
What it is
The embryonic period runs from week 3 to week 8 after fertilization (weeks 5 to 10 counted from the last menstrual period). In this time the three germ layers give rise to the primordia of every organ and the embryo turns from a flat disc into a recognizably human shape. That makes it the period of greatest risk for major structural malformations; afterwards, in the fetal period, growth and maturation dominate.
Key facts
- Week 3: gastrulation, notochord, neural plate, first somites, first vessels and blood cells.
- Around day 22: the heart tube starts beating.
- Week 4: folding, neural tube closure, pharyngeal arches, upper and then lower limb buds.
- Weeks 5–6: rapid head growth; the optic vesicle and placodes; physiological herniation of the midgut into the cord.
- Weeks 7–8: digital rays and separation of the fingers; the palate closes later, between weeks 7 and 10.
- End of week 8: about 3 cm crown–rump length; the external genitalia cannot yet be told apart.
What happens, in order
- Week 3: the bilaminar disc becomes trilaminar.
- Weeks 3–4: the neural tube forms and the neural crest migrates.
- Week 4: the embryo folds and ends up inside the amnion; the heart beats.
- Weeks 4–5: the pharyngeal arches and limb buds appear.
- Week 5: the brain forms five vesicles.
- Week 6: the midgut herniates physiologically into the umbilical cord.
- Weeks 7–8: the fingers separate and the face looks human.
Clinical notes
- Weeks 3–8 are the critical period for most teratogens (alcohol, isotretinoin, thalidomide, valproate, rubella).
- Each organ has its own window: the nervous system stays sensitive all pregnancy; the heart mostly in weeks 3–6.
- An insult in weeks 1–2 usually follows the all-or-none rule.
- First-trimester ultrasound shows cardiac activity at about 6 weeks' gestation.
Test yourself
Think of the answer, then tap the question to see it.
Which weeks make up the embryonic period, and why does it matter so much?
Weeks 3 to 8 after fertilization. Every organ forms then, so it is the period of greatest risk for major malformations.
When does the embryonic heart start to beat?
Around day 22 after fertilization, at the start of week 4.
How are weeks 3–8 after fertilization expressed as gestational age?
As weeks 5–10 counted from the first day of the last menstrual period.
Which part of the gut normally moves into the umbilical cord in week 6?
The midgut, in the physiological umbilical herniation; it returns to the abdomen around week 10.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Week 3: gastrulation and the trilaminar disc
- Neurulation and closure of the neural tube
- Embryonic folding: head, tail and lateral folds
- Heart tube: formation and looping
- Limb development: the apical ectodermal ridge, ZPA and limb rotation
- Midgut: physiological herniation and rotation
- Principles of teratology: critical periods, dose and genotype
- Fetal period: month 3 to birth
- Gestational age vs fertilization age: how pregnancy is dated
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. 6: Third to Eighth Weeks: The Embryonic Period (Wolters Kluwer, 2015); Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Gestational age: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.