39.  The three stages of prenatal development are:
a. Germinal stage  (Weeks 1-2) - Conception occurs and a zygote, which is a one-cell structure that is created when the sperm & egg unite, is reproduced.  Then the zygote is divided and multiplied through a process called mitosis.  These zygotes, after dividing, start forming the baby's organs and body parts, though, the cells have not yet attached to the lining of the mother's uterus.  The baby, at this point, has a genetic makeup and sex.  
b. Embryonic Stage (Weeks 3-8) - Once the process of mitosis has reached to approx 150 cells, it travels through the fallopian tubes and attaches itself to the lining of the uterus.  Once it is implanted, the organism is now an embryo.  The placenta is then formed to provide food and oxygen to the embryo through the umbilical cord.  During this stage, the heart begins to beat and the organs form and start to function.  At this point, the baby has a spinal cord and brain.
c. Fetal Stage (Weeks 9-40) - At this stage, the embryo is now a fetus and it is the size of a kidney bean.  The baby now takes the shape of a human being and the sex organs begin to differentiate.  The organs are developed enough that a premature baby can survive outside of the mother's womb.  The fetus continues to grow and gain weight until approximately 40 weeks.  The fetus, at this point, has little room to move around in the womb and is positioned for birth.