Pregnancy and Birth

Pregnancy is a normal physiologic process that begins with conception, follows through development and growth of the fetus and delivery, and ends with return to a fully normal state approximately 6 weeks after birth. Pregnancy causes physiologic changes in the mother's bodily functions to allow for growth and development of the fetus. For the fetus, pregnancy is a time of dependency on the mother for nutrition and thus, exposure to whatever agents to which the mother is exposed. Although a healthy pregnancy is normal for the majority of women, for some there may be complications that can lead to adverse outcome for the mother or the fetus.

The average biological length of human gestation from conception to delivery, is 266 days. Due to the difficulty in assessing the exact date of conception, however, the clinical length of pregnancy is considered to be 280 days, or 40 weeks, calculated from the last normal menstrual period before the cessation of menses, or menstrual flow. This calculation assumes that ovulation occurs 14 days after the last menstrual period. Human gestation a further divided into trimesters, each of which lasts slightly more than 13 weeks.

Fetal Development

After the ovum, or egg, is fertilized by a sperm, the fertilized ovum becomes implanted in the uterus.

First Trimester

Most fetal development, with the exception of such complex functions as brain development, occurs in the first trimester. The hart begins to beat after 4 weeks. By 8 weeks, the eyes, ears, nose, mouth, fingers, and toes are easily recognizable, and male or female reproduction systems have differentiated.

By 12 weeks, all of the recognizable organs have developed. During hese first weeks the fetus is most vulnerable to potential teratogenic, or birth-detect-inducing agents, such as drugs, radiation, and viruses. Drugs taken in by mother during the first weeks can be of particular harm.

Second Trimester

During the second trimester, thin walled skin to be formed in the bone to function, and blood begins to be formed in the bone marrow. In addition, scalp hair appears, subcutaneous fat increases, and bone start to harden. The fetus begins to move in the first trimester, and at about 20 weeks gestation the mother can perceive the movements, the onset of which is called "quickening."

Third Trimester

The majority of fetal weight gain occurs in the third trimester. Ear lobes begin to develop cartilage, testes start to descend into the scrotum, nails begin to grow over the tips of the digits, and creases develop over the soles of the feet. In addition, the fetus begins to demonstrate coordinated pattern of behavior.

The mother and fetus are physiologically connected via placenta, which filter oxygen and nourishment from the mother's blood to the baby via the umbilical cord. It also removes waste products from the fetus to the mother.