Beginning of pregnancy: Pregnancy begins at conception, which is also the starting point of gestational age. However, the organism does not become an embryo until about two weeks later, and pregnancy does not affect the woman (and cannot be confirmed by blood or urine testing) until implantation. Paradoxically, many obstetricians date the onset of pregnancy from the date of the woman’s last menstrual period (LMP), about 14 days before conception.

Length of pregnancy: Full-term pregnancies last 266 days, or 38 weeks, or 9 months. If the LMP is used as the starting time, pregnancy lasts 40 weeks, sometimes referred to as 10 lunar months (a lunar month is 28 days long).

Trimesters: Instead of germinal period, embryonic period, and fetal period, some writers divide pregnancy into three-month periods called trimesters. Months 1, 2, and 3 are called the first trimester; months 4, 5, and 6, the second trimester; and months 7, 8, and 9, the third trimester.

Due date: Although doctors assign a specific due date (based on the woman’s LMP), only 5 percent of babies are born on that exact date. Babies born between three weeks before and two weeks after that date are considered “full term” or “on time.” Babies born earlier are called preterm; babies born later are called post-term. The words preterm and post-term are more accurate than premature and postmature.