From the moment of conception, when an egg is fertilized, to the moment of birth, complex changes take place within a pregnant woman’s body. Genetic material from the father and the mother fuses together, eventually forming a new, genetically distinct individual. During this time, the mother provides a nourishing and protective environment in which the fetus can develop. When the baby is born, he or she is able to survive outside the mother’s body and begin a separate existence.
About a week after fertilization, the egg embeds itself securely in the lining of the uterus, at which stage it is known as an embryo. Only 8 weeks later, the embryo is recognizable as a human being and has developed all its vital organs, including the heart and brain. From this stage onwards, it is called a fetus. Between 8 and 12 weeks, the fetus grows rapidly, and its weight increases by up to 15 times. By the time the baby is born, it weighs just over 3 kg (7 lb) on average.
Changes in the mother’s hormone levels control the physical changes necessary for a healthy pregnancy and for birth. First, menstruation ceases, and then the ligaments and joints in the mother’s pelvis begin to soften and become more flexible in preparation for the birth of the baby. In addition, the mother’s breasts enlarge as their milk-producing glands increase in number, ready for breast-feeding the newborn baby.
Each new pregnancy runs a slightly different course. Many prospective mothers feel well throughout their pregnancies. Others experience uncomfortable symptoms such as nausea, vomiting, heartburn, indigestion, and tiredness. These are a result of the biological changes, such as pressure of the fetus on surrounding organs and hormone changes, occurring in the body as the fetus grows. Such symptoms develop at particular stages during the pregnancy.
After about 36 weeks, the fetus is fully formed and is able to survive independently outside its mother’s body. Around the 40th week, the first of the three stages of labour begins. After the baby is born, the mother begins to produce breast milk in preparation for breast-feeding, and her body gradually returns to its state before pregnancy.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.