The nine-month wait is finally over, and the baby is so longed for is about to be born. Gestation usually lasts between 37 and 42 weeks. But now her cervix thins, softens, and relaxes. The miracle of child birth begins. Several factors are involved in the process of child birth, but two are especially amazing.
First, oxytocin, a hormone produced in the brain, is released. Both men and women produce this substance, but a great quantity of it is released in the pregnant mother when labour begins, causing the cervix to dilate and the uterus to contract.
Just how the pituitary gland of the expectant mother knows when to begin releasing this hormone is a mystery. A second factor involved in this process is the role of the placenta, which stops producing progesterone. During pregnancy, progesterone has prevented strong contractions. But now, without the restriction of progesterone, the uterus begins to contract.
Generally after between 8 and 13 hours of labour, the baby is pushed out through the relaxed, dilated cervix. Afterward, the placenta is also expelled. Now the newborn must quickly adapt to new conditions of life, very different from those of its maternal environment.
For example, while in the uterus, the lungs of the fetus were full of amniotic fluid, which was squeezed out when the infant passed through the birth canal. Now the lungs must be filled with air to initiate breathing, the onset of which is usually indicated by the first cry. Drastic changes also occur in the heart and the rest of the circulatory system. A hole connecting the two atria of the heart and blood vessel bypassing the pulmonary circulation close, in order to reroute the blood through the lungs, thus enabling the blood to absorb oxygen.
