The First Trimester
Healthcare professionals divide a pregnancy into three trimesters. During the first trimester, or the first 12 weeks of pregnancy, your baby goes from a separate egg and sperm cell to a recognizable boy or girl with skin and bones that may even be sucking his thumb. Other developments during this stage are the development of a heart beat and the umbilical cord. Your baby develops a heartbeat at about five weeks, and arm and leg buds at the same time. In the sixth week, the baby begins to develop facial features. Eight weeks marks the beginning of the skeletal system's development. The bone starts as cartilage and continue to harden, both in utero and once the baby is born. At week 10, the baby will begin to use the placenta to draw in nutrients and excrete waste. According to Pregnancy.org by the end of the first trimester, your baby is approximately 3 ½ inches long and weighs approximately 1 ½ ounces.
The Second Trimester
The second trimester, weeks 13 through 27, is one of the most exciting times in the pregnancy. It is during this time that you may begin to feel your baby move. She has eyebrows, and her finger and toenails as well as the hair on her head are growing. At week 16, your baby will begin to develop a layer of fat under her skin. The development of reflexes has occurred, and your baby can suck, hiccup, swallow and blink. She is also practicing breathing, inhaling and exhaling amniotic fluid. By the end of the second trimester, your baby will respond to stimuli outside the womb, such as a bright light. According to Pregnancy.org, by the end of the second trimester, the average fetus will weigh nearly two pounds and be around 14.4 inches long.
The Third Trimester
You're in the homestretch of pregnancy now. This is a period of rapid growth as your baby prepares to enter the world. Most of the major developments are complete by this time, and your baby simply needs to put on weight and allow his organs to mature. His eyes are fully formed, and he is body is padding up with fat to help him adjust to life outside the womb. Your baby is taking over production of his red blood cells, with fully functional bone marrow. By the 36th week of pregnancy, nearly all of your baby's organs are fully functional. The one lagging body part is the lungs, and a baby born at this time may experience breathing difficulties. By 40 weeks, the average baby weighs 7.6 pounds and is 20.2 inches long, according to Pregnancy.org.
