How Does the Female Reproductive System Work?

To understand how reproductive hazards can affect a woman’s reproductive health and her ability to have healthy children, it is helpful to understand how the female reproductive system works. The main reproductive tissues in women are the ovaries, uterus, and fallopian tubes. The functions of these tissues are largely controlled by hormones produced by the brain and the pituitary gland. Hormones are chemicals that are formed in the body and circulate in the blood. These hormones regulate the menstrual cycle, pregnancy, and the production of breast milk. Estrogen and progesterone—the sex hormones—are produced by the ovaries. These hormones are responsible for sexual development and for preparing the uterine wall to hold and nourish a fertilized egg every month. These sex hormones also contribute to the basic health of the heart, bones, liver, and many other tissues. Finally, during pregnancy the placenta produces a hormone (human chorionic gonadotropin, or hCG) that signals the body to support the pregnancy. A woman is born with all of the eggs that she will ever have. Therefore, if her eggs are damaged or destroyed, she will not be able to replace them. At puberty, a woman begins to have menstrual cycles, which enable her to release an egg each month from one of her ovaries. Each cycle begins with a few days of menstrual flow. When each new cycle begins, a new egg starts to grow. After 2 to 3 weeks, a mature egg (ovum) is released from the ovary into the fallopian tubes, where it might be fertilized by one of the many sperm that may surround it. If the egg is not fertilized, it will die and leave the body about 2 weeks later in the woman’s menstrual flow. Then the process begins again with a new menstrual cycle. If the egg is fertilized, the complex process of reproduction continues. The fertilized egg travels for about a week down the fallopian tube to reach the uterus, where it attaches to the wall. A specialized tissue called the placenta forms between the uterus and the newly developing fetus. The placenta transfers oxygen and nutrients from the mother to the fetus. During the first 3 months of pregnancy (first trimester), the major fetal organs are formed. During the remainder of the pregnancy, these organs mature and the fetus grows rapidly.