The Due Date

A due date is important in pregnancy because it helps your doctor determine when to perform certain tests or procedures. It also helps estimate baby's growth and may indicate whether you are overdue or in premature labor.

Most women don't know the exact date their baby was conceived, but they usually know the day their last menstrual period began. ("Last menstrual period" is often abbreviated as "LMP.") Because ovulation occurs about the middle of your cycle, or about 2 weeks before the beginning of the next period, a doctor uses the first day of the last period and adds 2 weeks as an estimate of when conception occurred. By doing this, he or she can predict your due date.
You can also calculate when your baby is due in another way.
Begin with the date of the first day of your last menstrual period; add 7 days and count back 3 months. This gives you the approximate date of delivery. For example, if your last period began on January 20, your estimated due date is October 27.

Gestational age (menstrual age) is one way to date a pregnancy. If your doctor says you're 12 weeks pregnant, he's referring to gestational age. Your last menstrual period began 12 weeks ago, but you actually conceived 10 weeks ago.

Fertilization age (ovulatory age) is another way. It is 2 weeks shorter than gestational age and dates from the actual day of conception. This is the age of the fetus. In the case of "12 weeks pregnant," the fetus is actually 10 weeks old.

Your doctor may also refer to your pregnancy by trimesters.
Trimesters divide pregnancy into three stages, each about 13 weeks long. This division helps organize stages of development.