Pregnancy Trimester Calculator

Week 12 is the LAST week of the first trimester. The second trimester starts at week 13 (officially week 14 by some definitions). This is why many women wait until 12 weeks to share pregnancy news — miscarriage risk drops significantly after.

Week 20 is the second trimester. The mid-pregnancy anatomy ultrasound typically happens between weeks 18–22 in the second trimester.

Week 25 is still the second trimester. The third trimester begins at week 27 (some sources say week 28).

Week 26 is the LAST week of the second trimester. The third trimester begins at week 27.

The third trimester begins at week 27 of pregnancy. It lasts until delivery — most healthy pregnancies deliver between weeks 37 and 42.

First trimester: weeks 1–13 (13 weeks). Second trimester: weeks 14–26 (13 weeks). Third trimester: weeks 27 until delivery (typically 13–15 weeks). Total pregnancy: 40 weeks counted from LMP.