Pregnancy is a time of rapid development for a baby who begins with only two cells and eventually forms into an entire person. The growth of a baby before birth involves the complex process of change in physical features each week over the course of a pregnancy. By the time labor and birth arrive; a baby typically has undergone the necessary development to prepare him for the world outside the womb.
Conception
Prenatal development begins with the fertilization of an egg by a sperm cell. The fertilized egg becomes a zygote and attaches itself to the inside lining of the uterus where it will continue to grow. This process occurs over the course of the first two weeks after conception, during a time when a woman may not know that she is pregnant. After implanting in the uterus, the zygote continues with cell division, growing larger and developing physically.
1st Trimester
The time following conception to three months gestation is considered the first trimester. The embryo is developing with three layers of tissues: the ectoderm, mesoderm and endoderm. The ectoderm is made of the brain, skin and nerve cells; the mesoderm contains the heart, bones, muscles and blood vessels; and the endoderm is made of the stomach, intestines, liver and lungs. The spinal cord forms, and the heart begins beating. As the first trimester continues, the embryo develops sensory organs, such as the eyes and ears. By the end of the first trimester, extremities are moving and may be about 4 inches long, according to Brown University.
2nd Trimester
During the second trimester, a fetus is able to move and kick and has reflex responses, such as sucking. She has a sleeping cycle and will show periods of activity during the time she is awake. Hair develops on her head and fine hairs called lanugo cover her skin. She may begin opening her eyes, and her body will develop more fat. By the end of the second trimester, a fetus may weigh about 3 lbs., according to the Ohio State University Medical Center.
3rd Trimester
The third trimester begins at week 28 of gestation. During this time, the eyelids of a fetus are no longer fused. He can open his eyes and his pupils react to light by dilating. His skin begins to change into that which he will have after birth. The fine hairs of lanugo fall out and a protective covering called vernix coats the skin. He is active. A mother should feel the baby moving and kicking freely. His weight increases, and the average baby gains approximately 1/2 lb. each week after 36 weeks, reports Georgia Health Info.
Birth Preparation
When a baby is close to delivery, she will have developed enough to prepare for life outside the womb. Her skull bones have grown, but they are not fused yet to allow passage through the birth canal. The lungs of a baby are one of the last physical developments. Until birth, she obtains oxygen through the placenta. Her lungs are filled with fluid that will be replaced with air after she is born. Labor and the birth process then close the chapter of her growth from the time of conception until she is born, according to the Parents website.
