How pregnancy weeks are counted
The American College of Obstetricians and Gynecologists (ACOG) says a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period.[3] It says pregnancy is assumed to start 2 weeks after that day, so 2 weeks are counted at the beginning when you are not yet pregnant.[3]
The UK's NHS also says pregnancy is dated from the first day of your last period.[4]
ACOG's Committee Opinion 700 says the due date is, by convention, 280 days after the first day of the last period.[5] It says a first-trimester ultrasound is the most accurate way to set or confirm how far along a pregnancy is.[5]
ACOG says only about 1 in 20 women give birth on their due date.[3] The NHS says most women go into labor within a week either side of the due date.[6]
How sources split the trimesters
Trimester weeks as each source gives them:[1][2][3][7][8]
| Trimester | OWH (US) | NHS (UK) | ACOG (US) | MedlinePlus (US) |
|---|---|---|---|---|
| First | Weeks 1 to 12 | Weeks 4 to 12 | First day of last period to 13 weeks 6 days | Not stated |
| Second | Weeks 13 to 28 | Weeks 13 to 27 | 14 weeks 0 days to 27 weeks 6 days | Weeks 14 to 28 |
| Third | Weeks 29 to 40 | Weeks 28 to 41 | 28 weeks 0 days to 40 weeks 6 days | Weeks 28 to 40 |
Read by trimester
Week pages
Tests by week
OWH lists prenatal tests by week, from the first visit to the group B strep test at weeks 36 to 37.[9]
When labor usually starts
MedlinePlus says labor usually starts between 37 and 42 weeks.[10] It says labor that starts before 37 completed weeks is preterm labor.[11]
Warning signs at any week
CDC lists urgent warning signs to watch for during pregnancy and in the year after birth.[12]
Questions families ask
How are pregnancy weeks counted?
ACOG says pregnancy is counted from the first day of the last menstrual period, and lasts about 40 weeks.[3] The NHS also dates pregnancy from the first day of the last period.[4]
How many trimesters are there?
OWH and the NHS both describe three trimesters.[1][2]
When does the second trimester start?
OWH and the NHS start it at week 13.[1][2] ACOG and MedlinePlus start it at 14 weeks.[3][7]
When does labor usually start?
MedlinePlus says labor usually starts between 37 and 42 weeks.[10]
Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages
Updated 2026-10-24: first published.
Sources
All sources were opened and checked on the date shown.
- Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
- NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
- NHS (United Kingdom). 4 weeks pregnant guide. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists (with AIUM and SMFM). Methods for Estimating the Due Date (Committee Opinion No. 700; reaffirmed 2025). Checked October 24, 2026.
- NHS (United Kingdom). 39 weeks pregnant guide. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your second trimester. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your third trimester. Checked October 24, 2026.
- Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Childbirth. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Preterm Labor. Checked October 24, 2026.
- Centers for Disease Control and Prevention (HEAR HER campaign). Urgent Maternal Warning Signs and Symptoms. Checked October 24, 2026.