Hormones, such as human chorionic gonadotropin (HCG), progesterone, and estrogen, play important roles in pregnancy and can also affect the digestive, circulatory, respiratory, urinary, and integumentary systems.
Between puberty, which represents the maturation of the genital organs, the development of secondary sexual characteristics, and the beginning of menstruation, and menopause, or the time in a woman’s life when fertility ends, pregnancy may occur if the egg is fertilized after ovulation. Fertilization happens in the fallopian tube shortly after the egg is released. The fallopian tube is a hollow tube between the ovary and the uterus that conducts eggs to the uterus during a woman's menstrual cycle. The fertilized egg, called a zygote, then makes its way to the uterus, where implantation may occur. Implantation is the stage of pregnancy when the fertilized egg adheres to the wall of the uterus. If implantation does not occur, pregnancy does not proceed. Most pregnancies are single, meaning one fetus develops, although multiple pregnancies, in which more than one fetus develops, can occur. Multiple pregnancies result in twins, triplets, or, more rarely, births of greater numbers of infants.
Pregnancy is divided into three stages called trimesters. In the first trimester (weeks 1 through 12), the placenta, a flattened, circular, vascular organ that connects the developing fetus, called an embryo, to the uterus, forms. The placenta releases human chorionic gonadotropin (HCG), a hormone that maintains the corpus luteum (remnant of a ruptured Graafian follicle after ovulation), which results in increased levels of progesterone during the first trimester of pregnancy. Progesterone in turn increases the density of blood vessels in the uterine lining, nourishing the developing fetus. The uterus grows, and the mother may experience symptoms such as nausea (called morning sickness) and breast tenderness. She may have frequent urination, headaches, fatigue, and decreased sexual desire as well. Because levels of HCG are high during pregnancy, it is the most common hormone examined by pregnancy tests. Relaxin is a hormone that causes changes to the cardiovascular system during the first trimester of pregnancy. It also initiates labor and facilitates delivery of the baby by dilating the cervix and increasing flexibility of ligaments and pubic bones. Relaxin induces increased blood flow from the heart, increased renal blood flow, and an increase in the size of blood vessels.
In the second trimester (weeks 13 through 27), the uterus continues to grow as the fetus grows. Symptoms affecting other organ systems, such as nausea, headaches, and fatigue, decrease. The mother gains weight as a result of decreased nausea, although she may experience indigestion. Frequency of urination typically increases. In the second trimester, "quickening" occurs—the mother is able to feel the fetus move. During the second trimester, most women begin to show the physiological changes most associated with pregnancy. The skin may develop stretch marks as the torso expands to accommodate the growing fetus. Preparatory contractions called Braxton-Hicks contractions can also occur. These contractions do not signify that labor (the process of childbirth from onset of contractions to delivery) has begun.The third and final trimester lasts from week 28 to birth, typically at week 40. In the third trimester, heartburn increases. Fluid retention may lead to swollen ankles and wrists. Because of the weight they carry in their abdomen, many women experience constant backache during the third trimester. The skin of the abdomen may darken during this trimester as well. Progesterone levels decrease during this trimester, while levels of estrogen and prolactin rise. Prolactin, a hormone that stimulates milk production, increases, readying the breasts for lactation. Many mothers will experience secretion from the mammary glands near the end of the third trimester. Levels of the hormone relaxin also spike, which thins the cervix in preparation for childbirth. In addition, relaxin initiates labor and facilitates delivery of the baby by dilating the cervix and increasing flexibility of ligaments and pubic bones.