First Trimester (Weeks 1 to 13)
While this portion of your pregnancy spans three months, it’s considered the shortest trimester. The reason? Many women don’t realize they’re pregnant for the first month. (Home pregnancy tests generally will not register a positive result until about week 4.)
Moreover, week 1 and week 2 are actually the weeks you ovulate and have your menstrual period. So, while the duration of pregnancy consists of 40 weeks, the countdown starts roughly two weeks before you officially become pregnant. (Confusing, we know.)
While you may experience some belly changes, this is usually due to pregnancy-related bloating and gas, not baby growth. Still, by the end of your first trimester, you may gain between 1 and 4½ pounds.