A human pregnancy starts when the male sperm fertilizes the ovum (egg) in a woman’s Fallopian tube, and it lasts, on average, 266 days. Contraception works by inhibiting the release of the ovum from the ovary (birth control pill, injectible, or subdermal implant), by impeding the release of sperm (vasectomy), by blocking sperm from entering the vagina or cervix (male or female condom, diaphragm, or cervical cap), or by blocking the Fallopian tubes (tubal ligation). Once conception takes place, the fertilized egg travels through the Fallopian tube into the uterus, where it implants about seven days later. The intrauterine device (IUD) impedes such implantation, and medications like mifepristone (RU486) causes the implanted embryo to abort.
A developing human is called an embryo between two and eight weeks after conception; thereafter it is called a fetus until delivery. Development of the major organs begins during the early embryonic period, and interference with this process may result in birth defects. Women taking harmful substances, or women with preexisting diseases like diabetes mellitus, are at increased risk for having babies with birth defects. Because the development of major organs begins during early pregnancy, often before a woman starts prenatal care or realizes that she is pregnant, preconceptional care is recommended for every woman of reproductive age.
Although most major organs are present at the end of the embryonic period, the development of their functions continues well into the fetal period, infancy, and early childhood. Interference with this process may lead to functional deficits. For example, undernutrition during this period of growth has been associated with increased risk for coronary heart disease, and maternal alcohol use during pregnancy has been linked to mental retardation and other birth defects.
Remarkable changes take place in a woman during pregnancy. The heart circulates 40 percent more blood volume to supply nutrients and oxygen to the growing baby, deeper breaths occur and an increased amount of harmful substances are cleansed through the kidneys. Digestion slows down for better absorption or nutrients, which may cause problems such as heartburn and constipation. The baby is sustained in the uterus by the placenta, which serves as the interface between maternal and fetal circulations. Hormones prepare the breasts for lactation, and the immune system is altered so that it does not reject the baby as a foreign body. While most healthy women make these adaptations readily, pregnancy can jeopardize the health, and sometimes the lives, of women who are less healthy and suffer increased stress to the system during pregnancy
Tags: Physiology, Physiology Pregnancy
