Estimate your baby's due date and see how far along you are, right now.
The last-period method above uses Naegele's rule, the formula most care providers start with: take the first day of your last period, add one year, subtract three months, and add seven days — which is the same as adding 280 days (40 weeks) for an average 28-day cycle. We adjust it slightly for cycles longer or shorter than 28 days, since ovulation shifts earlier or later with cycle length.
Conception-date and IVF/embryo-transfer methods work backward from the same 280-day window, just counted from a later, more precise starting point — which is why an IVF due date is often considered slightly more reliable than one based on last period alone.
Using IVF or a frozen embryo transfer? Our dedicated tool covers Day 3, Day 5 and Day 6 transfers with IVF-specific guidance. Try the IVF & FET Due Date Calculator →
Pregnancy is tracked in weeks by doctors, but most people still think in months. Because a "pregnancy month" isn't a clean four weeks, the mapping isn't perfectly even — here's the breakdown used above:
| Weeks | Month | Trimester |
|---|---|---|
| 1–4 | Month 1 | 1st |
| 5–8 | Month 2 | 1st |
| 9–13 | Month 3 | 1st |
| 14–17 | Month 4 | 2nd |
| 18–22 | Month 5 | 2nd |
| 23–27 | Month 6 | 2nd |
| 28–31 | Month 7 | 3rd |
| 32–35 | Month 8 | 3rd |
| 36–40 | Month 9 | 3rd |
Want to convert a specific week, either direction, with a full explanation? Open the Weeks-to-Months Converter →
Only around 5% of babies are born on their exact due date — most arrive within about two weeks either side of it, and first-time pregnancies often run a little past 40 weeks. Due dates are best understood as the middle of a normal range, not a deadline.
Naegele's rule is the standard formula for estimating a due date from the first day of your last period: add one year, subtract three months, and add seven days. It assumes a 28-day cycle with ovulation on day 14, so care providers adjust it when your cycle runs longer or shorter, or once an ultrasound gives a more precise measurement.
Roughly: weeks 1–4 is month 1, weeks 5–8 is month 2, weeks 9–13 is month 3, and so on in slightly uneven four-to-five-week blocks through month 9 at weeks 36–40 — see the full table above, or use the dedicated converter for any specific week.
Yes — select "IVF transfer" above and choose whether it was a Day 3 or Day 5 transfer; the calculator adjusts for the embryo's age at transfer automatically. For Day 6 transfers and more detailed IVF/FET guidance, use the dedicated IVF & FET Due Date Calculator linked above.
As a rough rule of thumb, add about 9 months and a week to your conception month to land on your due month — for example, conceiving in April points to a due date in early-to-mid January. For an exact date rather than just a month, enter your conception date above, or see the full month-by-month table on the Reverse Due Date Calculator.
The math itself is the same 280-day estimate regardless of how many babies you're carrying — check "Expecting twins" above and we'll add a note about typical delivery timing, since twin pregnancies commonly arrive earlier than the calculated due date, often around 36 weeks rather than 40.