A pregnancy is divided into three stages. These are called trimesters. Each trimester has its own major milestones. The first trimester is 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 three 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.

The normal length of pregnancy is 37 to 41 weeks. Some babies may be born before 37 weeks or after 41 weeks. Maturity of the newborn is as follows:

• Preterm is a baby born before 37 weeks.
• Early term is a baby born from 37 weeks to 38 weeks and 6 days.
• Full term is a baby born from 39 weeks to 40 weeks and 6 days.
• Late term is a baby born from 41 weeks to 41 weeks and 6 days.
• Postterm is a baby born after 42 weeks.