Pregnancy
Pregnancy starts when a male’s sperm fertilizes a female’s ovum (egg), and the fertilized ovum implants in the lining of the uterus (see Fertilization ; Reproductive System ). Because pregnancy changes a woman’s normal hormone patterns, one of the first signs of pregnancy is a missed menstrual period (see Menstruation ). Other symptoms include breast tenderness and swelling, fatigue, nausea or sensitivity to smells, increased frequency of urination, mood swings, and weight gain. Some women also experience cravings for unusual substances such as ice, clay, or cornstarch; this condition, called pica, can indicate a dietary deficiency in iron or other nutrients. By the 12th week of pregnancy many of these symptoms have subsided, but others appear. For example, a woman’s breasts usually increase in size, and her nipples darken. The most obvious symptom is weight gain; most physicians now recommend a gain of about 9 to 12 kg (about 22 to 26 lb) by the end of pregnancy. The first few months ...