A pregnancy is divided into 3 phases. These are called trimesters. Each trimester has its own major milestones. The first trimester is the most fragile period. It's when all major organs and systems are formed. Most birth defects and miscarriages happen during the first trimester. In the second and third trimester, the fetus is fully formed and grows and matures rapidly. The trimesters are divided as follows:
First trimester is 0 to 12 weeks.
Second trimester is 13 to 28 weeks.
Third trimester is 29 to 40 weeks.
Some experts use the 42-weeks method divided by 3 trimesters:
The first trimester is 0 to 12 weeks.
The second trimester is 13 to 27 weeks.
The third trimester is 28 to 42 weeks.