Pregnancy Trimesters

Pregnancy typically covers nine months, but not 36 calendar weeks. When you and your doctor establish your baby's due date, you'll be told that your due date (EDD) is 40 weeks from the first day of your last menstrual period (LMP). If you deliver on your due date, you are actually only 38 weeks pregnant then, because of the two weeks difference between your LMP and the fertilization of the egg.

Model of baby during development

Any pregnancy has three characteristic periods, different from one another, that are called trimesters, each covering roughly 3 months of the entire pregnancy period. You should expect different body changes during each of the trimesters, as well as different developmental changes for your baby.

During the first trimester (week 1 through week 12), the fertilized egg gradually grows into an embryo, which then develops most of its body organs (the embryonic stage). With the implantation of the fertilized egg into the wall of the uterus, there comes a series of hormonal (and later, physical) changes in your body. Around the 9th week of growth, the embryo, having reached a length of more than 1in (2.54 cm) may be called a fetus. The uterus will have grown from the size of a fist to about the size of a grapefruit.

It is during this time that the risk of miscarriage is greatest.

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