The First Trimester

Content provided by the Faculty of the Harvard Medical School

The First Trimester

Pregnancy lasts an average of 282 days, measured from the first day of your last menstrual period. In truth, because pregnancy length is traditionally measured from the first day of your last period, 14 days before you ovulated (produce an egg for fertilization), for 14 of the 282 days, you aren't yet pregnant. It is broken up into three stages of equal time called trimesters. A normal full-term pregnancy lasts 40 weeks from the last menstrual period. The first trimester includes weeks 0-13, the second trimester includes weeks 14-26 and the third trimester includes weeks 27-40. Although many speak of the nine months of pregnancy, doctors and midwives prefer to use weeks as reference points. This avoids the confusion of different months having different number of days.

The early, developing pregnancy is called an embryo. The first trimester is a time of astounding embryonic growth and progress. During this 90-day period in which the embryo becomes a fetus, several key events occur: Estrogen and progesterone (female hormones) levels increase and remain high to support the changes occurring in your body and in the embryo. Within the uterus, a portion of the pregnancy tissue develops into the placenta, which will supply oxygen and nutrients to the embryo and dispose of its waste products. A protective membrane called the amniotic sac forms around the baby and fills with fluid. While the embryo grows, and the placenta begins to produce pregnancy hormones, you may notice changes in both your body and the way you are feeling.

Last updated:	September 29, 2004
Reviewed By:	Faculty of Harvard Medical School