A pregnancy is divided into 3 phases, called trimesters. Each trimester has its own significant milestones. The first trimester is the most fragile period, during which all major organs and systems are formed. Most birth defects and miscarriages happen during the first trimester. During 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 authorities use the 42-weeks method divided by 3 trimesters with the 1st trimester, 0 to 12 weeks; the 2nd trimester, 13 to 27 weeks; and the 3rd trimester, 28 to 42 weeks.