Just as pregnancy takes both mother and child through an incredible series of physical changes, so do labor and delivery. Although the baby has well formed major organ systems by the first trimester of pregnancy, birth alters several of them dramatically. The mother goes through her own great bodily transformation.
The Baby's Circulatory System and Lungs
The lungs develop early in pregnancy, but remain deflated and contain amniotic fluid until birth and the severing of the umbilical cord. This is when the baby stops bringing nutrients and oxygen into her bloodstream through the placenta. As she inhales and exhales her first breath, her lungs clear and start working automatically.
Early in pregnancy, the fetus produces her own form of hemoglobin, the molecule that ferries oxygen through the bloodstream. Gradually, she replaces it with adult hemoglobin. After birth, the blood begins to flow quickly around the heart in a different pattern and to enter vessels in the lungs. Two no longer functional "holes" in the heart, the ductus arteriosus and the foramen ovale, close as the baby starts to breathe air.
The Liver and Gastrointestinal Organs
The fetus' gastrointestinal organs do not need to work very much. In late pregnancy, she begins making meconium, a greenish black stool she excretes soon after, sometimes before, and during birth. Fetal intestines are sterile. Within 48 hours of birth, they gain the more than 200 species of beneficial bacteria -- called the intestinal flora -- they need to function properly.
Before birth, the baby's liver stores iron and glycogen, a form of sugar. After birth, her liver adds the abilities to break down wastes and excessive, old, degraded red blood cells, and to create compounds that enable blood clotting.
Urinary System
At nine to 12 weeks post-conception, the baby's kidneys start to make urine. At birth, the kidneys gain their ability to balance fluids and vital chemicals called electrolytes. They filter the bloodstream more speedily.
Immune System
The placenta protects the fetus from many harmful microbes (germs) that infect the mother. The fetal immune system, which commences in the first trimester, is largely devoted to avoiding rejection of the mother as "foreign tissue." Although the fetus and mother share half their genes, they are two distinct organisms.
After birth, the baby has direct exposure to bacteria and other dangers. She develops the ability to fight infectious diseases herself, helped along by maternal antibodies she accumulates before birth and afterward through breastfeeding.
Temperature Regulation
During pregnancy, the baby's body heat is double that of an adult. Following birth, the child's body cools steeply. In response to this temperature change, the baby shudders and uses up brown fat, a kind of fat the body can convert easily into heat.
Maternal Changes
During pregnancy, the mother's uterus grows from the size of a pear to an organ large and strong enough to hold and then propel a full-term baby and placenta down and out the vagina. It starts returning to original size right after birth. Blood flow increases in the mother's breasts so that her milk supply will "come in" about 48 to 72 hours after the delivery.
