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Conception
Although many sperm cluster around the egg and try to penetrate its outer layer, only one of them will manage to burrow its way through the surface and fertilize the egg. Once this happens, the egg's outer layer thickens quickly to keep out other competing sperm, so that each egg can be fertilized by only one sperm. -
Embryo to Fetus
Around week 8, the embryo becomes a fetus. The kidneys, liver, brain, and lungs are all beginning to function. The fingers and toes are separate and the external genitalia are formed. -
Second Trimester Begins
By week 15, the fetus is around five inches long and weighs about two ounces. Although its eyelids are fused shut, its eyes are now sensitive to light. -
Half Way There ~ 20 Weeks
At week 20 -- halfway through the pregnancy -- the fetus is about six inches long and weighs about 10 ounces. Its digestive system produces meconium, a black, tar-like substance that will make up its first few bowel movements. The fetus is coated in a white greasy substance called vernix caseosa, which will protect its skin from the amniotic fluid in utero. -
End of the 2nd Trimester ~ 27 Weeks
Week 27 marks the end of the second trimester. By now, the fetus has grown to about 14 inches long, and weighs about 2 pounds. It is already starting to look like a newborn baby. If born at this point, the baby may survive, but would face potentially serious problems. -
32 Weeks
By week 32, the fetus can move its eyes back and forth and tell the difference between light and dark. It is gaining weight quickly -- about a half pound per week. At this stage in the pregnancy, the fetus weighs just over four pounds and measures 15 to 17 inches long. It may have a full head of hair as well as fingernails and toenails. -
Week 35
At week 35, the fetus measures between 15 and 18 inches long and weighs five-and-a-half to 6 pounds. The lanugo starts to fall off. The vernix caseosa, which protected its body in the amniotic fluid, also disappears. Its organs are almost fully developed. -
37 Weeks
At week 37 the fetus is considered full term. This means that if it were delivered now, it would be able to function outside the womb. It should weigh at least 6 pounds and measure between 19 and 20 inches. At any time between now and the onset of labor, the mucus plug which had blocked the entrance to the mother's cervix to prevent bacteria from entering will come out. This is a sign that labor is on its way. -
40 Weeks
Week 40 officially marks the end of the pregnancy, but only a small percentage of women actually deliver on their due date -- many are either early or late
