Completed weeks
Gestational age is written as completed weeks plus days: 20+4 means 20 weeks and 4 days. After 6 days comes the next week, so 20+6 is followed by 21+0.1 Saying “20 weeks pregnant” means 20 completed weeks, so you are in your 21st week.
Trimesters
By ACOG’s convention, the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.2 Some sources start the second trimester at 13 weeks; it is a labeling choice.
Weeks to months
There is no official conversion. A calendar month averages about 4.35 weeks, so 40 weeks is about 9.2 months from the last period, and about 8.7 months from conception. That’s why “nine months” is a rough approximation. The table divides the count into average calendar months.
| Month of pregnancy | Weeks + days (approximate) |
|---|---|
| Month 1 | 0+0 to 4+1 |
| Month 2 | 4+2 to 8+4 |
| Month 3 | 8+5 to 12+6 |
| Month 4 | 13+0 to 17+2 |
| Month 5 | 17+3 to 21+4 |
| Month 6 | 21+5 to 26+0 |
| Month 7 | 26+1 to 30+2 |
| Month 8 | 30+3 to 34+5 |
| Month 9 | 34+6 to 39+0 |
| Month 10 | 39+1 to 43+2 |
Month 10 covers the last weeks up to the due date and beyond. Because months vary, apps that use other conventions may differ by a week.
Examples
- 12 weeks 0 days: 12 completed weeks, in the 13th week, still first trimester, about the end of month 3.
- 27 weeks 6 days: the last day of the second trimester.
- 36 weeks 5 days: third trimester, still preterm by definition until 37 weeks 0 days.3