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Your Developing Baby, Week by Week

Your baby begins as a tiny cluster of cells, but during the nine-month process of pregnancy she becomes an amazing baby with facial features, a beating heart and kicking legs. Find out what's happening with baby's development week by week.
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Amazingly, your child's sex and all of her inherited genetic characteristics -- such as eye color, hair color, skin, and body type -- have been set since the moment of conception. Your developing baby, now called a zygote, has 46 chromosomes -- 23 from you and 23 from your partner. These chromosomes help determine your baby's sex and traits such as eye and hair color, and, to some extent, personality and intelligence. After fertilization, the ball of cells, now an embryo, will wrap up its journey through the fallopian tube and burrow itself into the wall of your uterus for nourishment -- a process known as implantation. If you're having multiples, the deed has already been done. Fraternal twins occur when two separate eggs are fertilized by two different sperm and each baby has his own placenta and amniotic sac. If one fertilized egg splits and develops into two fetuses, the result is identical twins. They may share a placenta, but each baby usually has a separate amniotic sac.