Anatomy chapters
The embryonic and fetal periods
Weeks three to eight, when every organ starts to form, and the fetal period, when the body grows and matures, with the timings exams test.
- The embryonic period (weeks 3 to 8) is when all major organs begin to form.
- It is the period most sensitive to teratogens.
- The fetal period (week 9 to birth) is mainly growth and maturation, measured by crown-rump length.
On this page
Overview
Development after fertilisation is split into two periods.
- Embryonic period: from the start of the third week to the end of the eighth week. All main organs start to form (organogenesis).
- Fetal period: from the ninth week to birth. Organs grow and mature.
The embryonic period is when the embryo is most at risk from drugs, infections and radiation.
The embryonic period (weeks 3 to 8)
Each germ layer starts to form its own tissues and organs (see the second and third weeks).
Folding of the embryo
The flat disc folds in two directions.
- Head-to-tail (cephalocaudal) folding: the head and tail ends curl under. The heart and the septum transversum move to the front of the chest region.
- Lateral folding: the two sides roll forward and meet in the front.
Folding has three big results:
- Part of the yolk sac is taken inside and forms the gut tube (foregut, midgut, hindgut).
- The embryo becomes a C-shaped cylinder.
- The amnion surrounds the embryo and the umbilical cord forms where the yolk sac and connecting stalk meet the body.
Lateral folding traps part of the yolk sac inside the embryo as the gut tube. Failure of the body walls to close causes defects like gastroschisis.
Key events by week
| Week | Main events |
|---|---|
| 3 | Gastrulation, notochord, neural plate; the heart tube starts |
| 4 | Folding; neural tube closes; heart begins to beat; limb buds and pharyngeal arches appear |
| 5 | Rapid growth of the head; hand plates form |
| 6 | Finger rays; external ear starts to form |
| 7 | Midgut herniates into the umbilical cord |
| 8 | Fingers and toes separate; the embryo has a human appearance |
The heart begins to beat at about day 22, early in week 4. It is the first organ system to work.
Why this period is sensitive
Organs are being laid down. A harmful agent at this time can cause a major structural defect.
Before week 3, a harmful agent usually either kills the embryo or does no harm ("all or nothing").
The first two weeks are usually not the time of major birth defects. The embryonic period (weeks 3 to 8) is.
The fetal period (week 9 to birth)
The fetus grows fast and its organs mature. The head grows more slowly than the body, so it becomes smaller in proportion.
| Month | Main changes |
|---|---|
| 3 | Face looks human; external genitalia can be told apart by about week 12; the midgut returns to the abdomen |
| 4–5 | Rapid growth in length; the mother feels movements (quickening); fine hair (lanugo) appears |
| 6 | Skin red and wrinkled; lungs begin making surfactant |
| 7 | Eyelids open; a fetus born now may survive with intensive care |
| 8–9 | Fat is laid down; skin covered in vernix caseosa; rapid gain in weight |
Length grows fastest in the third to fifth months. Weight increases most in the last two months.
Measuring age
- Crown-rump length (CRL): the best ultrasound measure of age in the first trimester.
- Later, the head, abdomen and femur length are used.
Fertilisation age is counted from fertilisation. Gestational (menstrual) age is counted from the first day of the last menstrual period. It is about 2 weeks more.
A full-term pregnancy is about 38 weeks after fertilisation or 40 weeks after the last period.
Clinicians use gestational age (from the last period). Embryology books often use fertilisation age. Check which one a question means.
Viability
A fetus becomes able to survive outside the uterus, with intensive care, from about the late second trimester. The lungs are the main limit, because they need enough surfactant and enough thin-walled air spaces.
Clinical relevance
Low birth weight and growth restriction
How it happens: poor placental function, maternal illness, smoking, poor nutrition or infection.
What you see: a baby small for its gestational age, at risk of low sugar and other problems after birth.
Preterm birth
How it happens: birth before 37 completed weeks.
What you see: breathing problems (respiratory distress syndrome) from lack of surfactant, plus feeding and temperature problems.
Birth defects
How it happens: a teratogen or genetic problem during weeks 3 to 8.
What you see: structural defects in the organ forming at that time. See teratology.
Know the start and end of the embryonic period, why it is the sensitive period, the results of folding, and the difference between fertilisation age and gestational age. The month when length grows fastest and when weight grows fastest are also asked.
Quick revision
- Embryonic period: weeks 3 to 8, organogenesis, highest risk of major defects.
- Fetal period: week 9 to birth, growth and maturation.
- Folding forms the gut tube and the C-shaped embryo.
- Heart beats from about day 22.
- Length grows fastest in months 3 to 5; weight in the last 2 months.
- Gestational age is about 2 weeks more than fertilisation age.
- CRL is the best early dating measure.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
What is the embryonic period?
Weeks 3 to 8 after fertilisation, when all major organ systems begin to form.
When does the fetal period start?
At the beginning of the ninth week.
Why is the embryonic period so sensitive?
Organs are forming, so a harmful agent can disturb their basic structure and cause major birth defects.
What is the difference between gestational age and fertilisation age?
Gestational age is counted from the first day of the last menstrual period and is about two weeks more than fertilisation age.