Following conception, a woman typically goes through three major states of pregnancy called trimesters, according to The Parent Site. These stages involve various growth aspects of the fetus as it develops through the gestational period during the usual nine-month pregnancy process. Although some babies are born prematurely, the majority undergo normal stages of development and growth in the womb.
First Trimester
The first trimester generally describes the first three months of pregnancy. During this stage, the embryo begins to develop its brain, heart, spinal cord and digestive system, according to Saving Angel Organization. By about the fifth week of development, the fetus grows arm and leg buds and the heart has developed a regular rhythm. At this stage, the eyes and ears are also developing, according to American Pregnancy Association. By the end of the fifth week, the baby's major organs have formed. By the sixth week, the bones are forming and facial features become more distinct. By the end of the 11th week, the baby has formed genitalia and the head has grown to nearly half its entire body size.
Second Trimester
The second trimester takes place between week 14 and week 23. Now about 6 inches in length, the baby may weigh about 4 oz. and be felt by the mother. The baby has grown finger- and toenails and her heartbeat is now audible through a stethoscope. Between the 21st and 23rd weeks, the baby is covered with a substance called vernix, which will protect baby's skin from the amniotic fluid surrounding it. By the 21st week, the baby may be approximately 11 inches in length. The eyes are fully developed, though they remain closed, and the liver and pancreas are completing development.
Third Trimester
The third trimester begins at week 27. The baby is adding and storing fat and may be almost 17 inches long, weighing about 4 lbs. Although the lungs are still developing, the bones are fully developed, though still soft. At this stage, the baby's eyes may open and the fetus will start to make its way into a head-down position in preparation for delivery. The fetus grows quickly during this stage, often growing to between 19 and 21 inches in length and gaining a few additional pounds. At nearly 37 weeks, the baby is crowding the uterus and the organs are fully developed, though the lungs will continue to mature until delivery, according to American Pregnancy Association.
