Popular and professional books use various phrases to segment the stages of pregnancy. The following comments may help to clarify the phrases used.
- 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 is not confirmed by blood or urine testing) until implantation. Perhaps because the exact date of conception is usually unknown, some obstetricians and publications count from the woman’s last menstrual period (LMP), usually 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 expressed as 10 lunar months. (A lunar month is 28 days long.)
- Trimesters: Instead of germinal period, embryonic period, and fetal period, as used in this text, 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 a specific due date based on the LMP is calculated, only 5 percent of babies are born on that exact day. Babies born between three weeks before and two weeks after that date are considered full term, although labor is often induced if the baby has not arrived within 7 days of the due date. Babies born more than three weeks early are preterm, a more accurate term than premature.
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