Timeline of human prenatal development


Changes by weeks of gestation

Gestational age vs. embryonic age

Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.
Thus, the first week of embryonic age is already week three counting with gestational age.
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.
The following table summarizes the various expression systems during week number x of gestation.
Week
number		Initial age
Gestationalxx-1
Embryonicx-2x-3

Week 3

Gestational age: 2 weeks and 0 days until 2 weeks and 6 days old. 15–21 days from last menstruation.
Embryonic age: Week nr 1. 0 weeks old. 1–7 days from fertilization.
Gestational age: 3 weeks and 0 days until 3 weeks and 6 days old. 22–28 days from last menstruation.
Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.
Gestational age: 4 weeks and 0 days until 4 weeks and 6 days old. 29–35 days from last menstruation.
Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
, still in the oviduct. This embryo is about five weeks old.

Week 6

Gestational age: 5 weeks and 0 days until 5 weeks and 6 days old. 36–42 days from last menstruation.
Embryonic age: Week nr 4. 3 weeks old. 22–28 days from fertilization.

Week 7

Gestational age: 6 weeks and 0 days until 6 weeks and 6 days old. 43–49 days from last menstruation.
Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.
, this one in the cornu. The features are consistent with
a developmental age of seven weeks.

Week 8

Gestational age: 7 weeks and 0 days until 7 weeks and 6 days old. 50–56 days from last menstruation.
Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.
Gestational age: 8 weeks and 0 days until 8 weeks and 6 days old. 57–63 days from last menstruation.
Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.

Weeks 10 to 12

Gestational age: 9 weeks and 0 days until 11 weeks and 6 days old.
Embryonic age: 7 weeks and 0 days until 9 weeks and 6 days old.
Gestational age: 12 weeks and 0 days until 15 weeks and 6 days old.
Embryonic age: 10 weeks and 0 days until 13 weeks and 6 days old.

Week 21

Gestational age: 20 weeks old.
Embryonic age: 18 weeks old.
Gestational age: 22 weeks old.
Embryonic age: 20 weeks old.
Gestational age: 24 weeks old.
Embryonic age: Week nr 25. 24 weeks old.
Gestational age: 30 weeks old.
Embryonic age: Week nr 29. 28 weeks old.
Gestational age: 34 weeks old.
Embryonic age: Week nr 33. 32 weeks old.

Weeks 36 to 40

Gestational age: 35 and 0 days until 39 weeks and 6 days old.
Embryonic age: Weeks nr 34–38. 33–37 weeks old.