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Fetal Development

This information is intended for general education purposes only and should never be relied upon as a substitute for professional medical advice.


Pregnancy is counted form the first day of a woman's last period. This means that at conception, the unborn child is already considered two weeks old.

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 embryo may float freely in the uterus for about 48 hours before implanting. Upon implantation, 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
The fetus is now 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 ½ 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 over a 50% chance of surviving outside of the womb.

Week 30
The fetus is usually capable of living outside of 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 outside of his mother's womb.

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 using this table, subtract two weeks.

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