Development of the Embryo and Fetus
Time	Stage of Development	Trimester
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 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 over a 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.

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.

Source: Introduction to Child Development, 6th ed., John Dworetzky, West Publishing Company, San Francisco, CA, 1996, p. 70-75.

Photo on Right: Arms and hands develop earlier than legs and feet, but by two months, the lines of palms and footprints are already engraved in the skin.