[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) › Weeks 3–8: the embryonic period and organogenesis [Embryology](https://durgical.com/en/embryology/) · [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) # 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](https://durgical.com/en/anatomy/spinal-cord/) ## Key points - Organogenesis: weeks 3–8 after fertilization (5–10 after the last period). - Highest risk of structural malformations. - The heart beats around day 22; the neural tube closes around day 28. - Limb buds appear at the end of week 4. - From week 9 it is called a fetus. ## 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. ## Memory aid Weeks 3 to 8 are the body's 'framing stage': every foundation and wall goes up; the fetal period only finishes, enlarges and paints. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/weeks-3-8-the-embryonic-period-and-organogenesis/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) ## Related - [Week 3: gastrulation and the trilaminar disc](https://durgical.com/en/embryology/week-3-gastrulation-and-the-trilaminar-disc/) - [Neurulation and closure of the neural tube](https://durgical.com/en/embryology/neurulation-and-closure-of-the-neural-tube/) - [Embryonic folding: head, tail and lateral folds](https://durgical.com/en/embryology/embryonic-folding-head-tail-and-lateral-folds/) - [Heart tube: formation and looping](https://durgical.com/en/embryology/heart-tube-formation-and-looping/) - [Limb development: the apical ectodermal ridge, ZPA and limb rotation](https://durgical.com/en/embryology/limb-development-the-apical-ectodermal-ridge-zpa-and-limb-rotation/) - [Midgut: physiological herniation and rotation](https://durgical.com/en/embryology/midgut-physiological-herniation-and-rotation/) - [Principles of teratology: critical periods, dose and genotype](https://durgical.com/en/embryology/principles-of-teratology-critical-periods-dose-and-genotype/) - [Fetal period: month 3 to birth](https://durgical.com/en/embryology/fetal-period-month-3-to-birth/) - [Gestational age vs fertilization age: how pregnancy is dated](https://durgical.com/en/embryology/gestational-age-vs-fertilization-age-how-pregnancy-is-dated/) [← 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: Langman's Medical Embryology, 13th ed., ch. 6: Third to Eighth Weeks: The Embryonic Period (Wolters Kluwer, 2015); [Fetal development: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002398.htm) (MedlinePlus (U.S. National Library of Medicine), 2026); [Gestational age: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002367.htm) (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. --- Source: https://durgical.com/en/embryology/weeks-3-8-the-embryonic-period-and-organogenesis/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/weeks-3-8-the-embryonic-period-and-organogenesis/