Estimate your due date and current week of pregnancy from the first day of your last menstrual period.
This calculator uses Naegele's rule, the standard method most care providers use for an initial estimate: it adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than the typical 28 days, the calculator adjusts slightly for that difference, since ovulation timing shifts along with cycle length.
Pregnancy is conventionally dated from the first day of your last period, not from the day of conception — which usually happens about two weeks later. That's why in week 4, for example, you're often only about two weeks past actual fertilization. This can feel confusing at first, but it's the standard convention doctors and ultrasounds use.
| Trimester | Weeks |
|---|---|
| First | 1–13 |
| Second | 14–27 |
| Third | 28–40+ |
Only about 5% of babies are born on their exact estimated due date — it's a helpful planning marker, not a precise prediction. Always confirm dates and milestones with your doctor or midwife, especially if your cycle is irregular.