Find your current week and trimester from either your last period or your due date.
Estimates only, not medical advice.
Pregnancy is measured in weeks in clinical settings, but everyday conversation still leans on "months" and "trimesters," and none of those three units divide evenly into the others. A term pregnancy runs about 280 days, roughly 9 and a half calendar months, or 10 lunar months of exactly 4 weeks each. The common shorthand of "nine months" is an approximation that has stuck around even though clinicians work in weeks because weeks are more precise.
| Unit | Approximate length of a term pregnancy |
|---|---|
| Weeks | 40 weeks |
| Calendar months | About 9.2 months |
| Lunar months (4 week blocks) | 10 months |
| Trimesters | 3, of unequal length |
The three trimesters used here follow the standard clinical convention: first trimester through week 13, second trimester weeks 13 through 27, third trimester from week 27 to birth. Notice these are not three equal 13.3 week blocks; the boundaries group pregnancy into broadly similar phases of development and typical prenatal care rather than dividing 40 weeks evenly. Your provider uses the same three way split.
This calculator accepts either a last menstrual period date or a due date, and both routes use the same 280 day span in opposite directions: enter an LMP and it adds 280 days to find the due date, enter a due date and it subtracts 280 days to find an equivalent LMP. Either way, the resulting week count, trimester, and days remaining stay consistent with each other, since they are all derived from the same underlying timeline.
Forty weeks works out to roughly 9 and a half calendar months, or 10 four week lunar months, which is why the common shorthand of nine months is only an approximation.
The boundaries follow the clinical convention used across obstetric care rather than a strict division of 40 weeks into three equal parts. It groups pregnancy into broadly similar phases rather than exact thirds.
The calculator works backward or forward using the standard 280 day span, so either starting point produces a consistent week count and trimester.
If a dating scan shifts your estimated due date, your calculated week and trimester will shift with it.