A man's sperm in the woman's fallopian tubes fertilizes an egg. It takes a few days for the fertilized egg to move through into the uterus where it will grow. The implanted cells become the fetus and placenta, providing nutrients, oxygen and hormones. For the next 36 to 40 weeks, a fetus develops into a fully formed baby.
First Three Months
While the initial fertilized cells divide into the fetus and placenta, tiny arms form on the baby. The brain, spinal cord, lungs and heart begin to develop and, by the end of the first month, a heartbeat can be detected. Through the second month, eyelids and ear canals form as well as fingers and toes. Bones and genitals appear. By the third month, all major organs are in place. At just three months, a fetus should weigh about an ounce and measure nearly 3.5 inches. Appendages, internal organs and muscles continue to develop. Future teeth are marked by 20 new buds in the mouth and the baby's skin is nearly transparent.
Second Three Months
As fetal development moves into the fourth month, the baby's legs and arms begin to move slightly. External genitals, eyelashes, eyebrows and fingernails appear. During the fourth month, the baby's kidneys are formed and she begins to produce urine. Hearing and swallowing begin and the neck is formed. At about 10 inches and between 1.5 and 1 pound, the fetus wakes and sleeps on a regular basis during the fifth month. The baby becomes much more active and begins sucking, sometimes putting her thumb in her mouth. Hair begins to grow, replacing the soft fuzz that covered the outer layer of skin. During the sixth month, the baby's brain is in rapid development and her eyes open and close. She's got fingerprints at this time and, though she doesn't make noise yet, her lungs are fully developed.
Last Three Months
The kicking and stretching really take off in the seventh month as the baby reacts more often to outside stimulus. During the last two months of pregnancy, a fetus grows rapidly and can gain from 1 to 4 pounds and grow an additional 2 to 3 inches. Bones harden, and the skull becomes softer in preparation for delivery. Different areas of the brain develop during the eighth month, and the fetus becomes able to taste sweet and sour. Prior to delivery, the fetus begins to get into position sometime during the ninth month by moving her head down toward the birth canal. Her skin smooths out, and she is ready to scream once she appears.