Embryology · Fetal period, placenta and twins
Fetal period: month 3 to birth !
- Stage
From week 9 after fertilization the embryo is called a fetus: its organs are already laid down, and from now on it mainly grows and matures.
Updated 2026-11-01
What it is
The fetal period runs from the start of week 9 after fertilization to birth. The organs are already in place, so rapid growth and tissue maturation dominate. That is why harmful exposures at this stage seldom cause major malformations but do affect growth, the brain and organ function.
Key facts
- Length is measured as crown–rump length (sitting height) or crown–heel length (standing height).
- At the end of month 5 the fetus still weighs less than half a kilogram.
- First fetal movements (quickening) are usually felt between weeks 16 and 20 of gestation, later in a first pregnancy.
- Around weeks 26 to 28 of gestation the eyelids open and the fetus responds to loud sounds.
What happens, in order
- Month 3: the face looks human, the eyes move to the front and the intestinal loops return to the abdomen.
- Month 3: primary ossification centres appear and, by about week 12, the external genitalia can be told apart on ultrasound.
- Months 4–5: the fetus lengthens fast, is covered in lanugo, and the mother starts to feel movements.
- Month 6: reddish, wrinkled skin for lack of fat; the lungs cannot yet sustain effective breathing.
- Month 7: the eyes open, terminal sacs form in the lung and survival outside the uterus improves greatly.
- Months 8–9: subcutaneous fat builds up, vernix caseosa covers the skin and, in boys, the testes descend into the scrotum.
- Birth: about 266 days after fertilization; the average term fetus weighs about 3 kilograms.
Clinical notes
- Prematurity: birth before week 37 interrupts the maturation of lung, brain, kidney and gut.
- Fetal age is estimated from the last menstrual period and ultrasound; first-trimester crown–rump length is the most accurate measure.
- Growth restriction and macrosomia are problems of the fetal period, not the embryonic period.
Test yourself
Think of the answer, then tap the question to see it.
In which week after fertilization does the fetal period begin?
At the start of week 9 after fertilization, which corresponds to week 11 of gestation counted from the last menstrual period.
In which part of the fetal period is most weight gained?
In the last two and a half months, when about half of term weight is added, mostly as fat.
Why does a fetus born early in month 6 struggle to survive?
Because the respiratory system and central nervous system are not yet differentiated or coordinated enough to breathe outside the uterus.
What share of the body does the head make up at birth?
About a quarter of body length, compared with half of the crown–rump length at the start of month 3.
Related
- Weeks 3–8: the embryonic period and organogenesis
- Gestational age vs fertilization age: how pregnancy is dated
- Fetal lung, kidney and brain maturation: what preterm birth interrupts
- Fetal growth and growth restriction
- Placenta: structure, circulation and functions
- Principles of teratology: critical periods, dose and genotype
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: Fetal development: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.