The three stages of prenatal development occur in the order of germinal stage, embryonic stage and fetal stage, according to About.com. The germinal stage lasts just two weeks. The embryonic stage lasts six weeks, and the fetal stage is the longest, occurring from the ninth week of pregnancy until the baby is born.
During the germinal stage, About.com explains that a woman's egg becomes fertilized within one of two Fallopian tubes and then travels down to the uterus where it attaches itself and remains until full term. Once the egg has implanted itself within the uterus, a woman's hormones respond to its presence and cause her regular menstrual cycle to stop for the remainder of the pregnancy. In the embryonic stage, the embryo begins to develop a neural tube, a head, eyes, a nose, ears and a mouth. A faint heart pulse forms, as well as stumps where the arms and legs later develop. During the fetal stage, the neural tube turns into a brain, the blood vessels that formed a pulse develop into a heart and sex organs begin to differentiate between male and female. The fetus begins to develop fingernails, hair and eyelashes. The brain develops quickly and the fetus grows substantially in size until the ninth month, when it is finally ready to be delivered.