Conception, when the sperm fuses with the egg, usually occurs about two weeks after the beginning of the last menstrual period (LMP). This section discusses your baby's development during weeks 13 through 25 from conception and not from the LMP.
Weeks Thirteen Through Sixteen
This is a period of rapid growth for the fetus. The arms and legs grow longer and the fetus begins to look more human. The eyes move to the front from the sides of the head. The fetus' bones are now detectable by x-ray.
Weeks Seventeen Through Twenty
Growth begins to slow during these weeks. Fetal movement is now detectable by the mother (a sensation called quickening), although first time moms take longer to feel fetal movement than women who have had babies already. The fetus at this point has little fat. The skin is very thin and is covered with a greasy substance called vernix, which protects it from drying out. In the latter half of pregnancy, the body is covered in a fine hair called lanugo, which helps to hold the vernix in place.
Weeks Twenty-One Through Twenty-Five
Now the fetus starts to gain more weight and fill out. Fingernails begin to form, although the skin is still wrinkly and almost transparent. The skin is red because of all the small blood vessels underneath the skin.
Around 24 weeks, the baby's lungs begin to produce surfactant, which is critical to the baby's survival if born premature. Premature babies born around this time may die due to a lack of surfactant in their underdeveloped lungs.