Pregnancy is the time (usually nine months) during which a woman carries one or more offspring until delivery. Childbirth usually occurs about 38 weeks after the conception, or about 40 weeks after the last menstrual period. The World Health Organization defines the normal term to be 37 to 42 weeks. During your first OB/GYN doctor visit, the doctor will normally tell you the estimated childbirth date, or the "due date", mainly interpreted by a sonograph measurement. The actual length of pregnancy depends on various factors. For example, the first pregnancy tends to last longer than subsequent pregnancies. Fewer than 10% of births occur on the due date; 50% of births occur within a week of the due date, and almost 90% occur within two weeks of the due date. The 40 weeks of pregnancy is typically broken down into three periods, or trimesters, each consisting of about three months.
Pregnancy can be detected via pregnancy tests, which detect your hormone level. Clinical blood and urine tests can detect pregnancy from six to eight days after fertilization. Blood tests are more accurate. Home pregnancy tests are urine tests, which normally can detect pregnancy from 12 to 15 days after fertilization.