Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-26), and third (weeks 27-40). Each involves distinct developmental milestones for the baby and physical changes for the mother.
The Three Trimesters
- First Trimester (Weeks 1-12): Embryo develops major organs, heartbeat begins (week 6), most critical period for avoiding harmful exposures. Common symptoms: nausea, fatigue, breast tenderness.
- Second Trimester (Weeks 13-26): Often called the "golden trimester." Baby grows rapidly, movements are felt (weeks 18-22), anatomy scan reveals sex. Morning sickness typically subsides.
- Third Trimester (Weeks 27-40): Baby gains weight and lung maturity. Braxton-Hicks contractions begin. Labor preparation starts. Baby is full-term at 39 weeks.
Real-World Example
A pregnancy with a last menstrual period of January 1: First trimester ends around March 26. Second trimester ends around June 25. Due date: approximately October 8 (40 weeks from LMP). Each trimester spans roughly 13 weeks.
Frequently Asked Questions
When does the second trimester start?
The second trimester begins at week 13 (the start of month 4). This is approximately 3 months after the first day of your last menstrual period.
Which trimester is most critical?
The first trimester is the most critical for development, as all major organs form during this time. The risk of miscarriage is highest in the first trimester. After 12 weeks, the risk drops significantly.
How is a due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. Only about 5% of babies are born on their exact due date; most arrive within 2 weeks before or after.