Fetal Development

Obstetricians count "weeks of pregnancy" from the first day of a woman's last menstrual cycle because there is often no way to determine exactly when conception occurred. Embryologists, however, typically describe the developing embryo or fetus by the number of weeks since conception. To determine the age of the unborn child since conception, subtract two weeks from the ages below:

Week 2

Conception is the moment at which the sperm penetrates the ovum. Once fertilized, it is called a zygote, until it reaches the uterus 3-4 days later.

Week 4

The zygote may float freely in the uterus for about 48 hours before implanting. Upon implantation, it is called an embryo, and complex connections between the mother and embryo develop to form the placenta.

Week 6

The embryo is about 1/5 of an inch in length. A primitive heart is beating. Head, mouth, liver, and intestines begin to take shape.

Week 10

The embryo is now about 1 inch in length. Facial features, limbs, hands feet fingers and toes become apparent. The nervous system is responsive and many of the internal organs begin to function.

Week 14

Now called a fetus, it is 3 inches long and weighs almost an ounce. The muscles begin to develop and sex organs form. Eyelids, fingernails, and toenails also form. The child's spontaneous movements can be observed.

Week 18

The fetus is now about 5 inches long. The child blinks, grasps, and moves her mouth. Hair grows on the head and body.

Week 22

The fetus now weighs approximately 1/2 a pound and spans about 10 inches from head to toe. Sweat glands develop, and the external skin has turned from transparent to opaque.

Week 26

The fetus can now inhale, exhale and even cry. Eyes have completely formed, and the tongue has developed taste buds. Under intensive medical care, the fetus has a greater than 50% chance of surviving outside the womb.

Week 30

The fetus is usually capable of living outside the womb and would be considered premature at birth.

Week 40

This marks the end of the normal gestational period. The child is now ready to live in outside of his mother's womb.

first trimester
second trimester
third trimester
labor & delivery