Due Date Calculator
How this is calculated
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 weeks to months
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 →
Common edge cases
The calculator's cycle-length field only accepts 20–45 days, but real cycles sometimes fall outside that range — very short cycles (under 21 days) and long cycles (over 45 days) are both fairly common with conditions like PCOS, thyroid changes, perimenopause, or in the months right after stopping hormonal birth control. If that's you, the LMP method will be less reliable, and it's worth using the conception-date method instead if you have a good estimate of when ovulation happened.
Naegele's rule also assumes ovulation happens a fixed number of days before the next period, but ovulation timing genuinely varies cycle to cycle, even for people with regular periods — stress, illness, travel, and breastfeeding can all delay it. A due date based on a last period is always a starting estimate, not a fixed appointment.
If an early ultrasound (usually done in the first trimester, most accurate between about 8 and 13 weeks) gives a different due date than the one calculated here, most care providers will use the ultrasound date going forward. Ultrasound measures the size of the fetus directly, which tends to be more precise than working backward from cycle assumptions, especially for people with irregular cycles.
The three methods above — last period, conception date, and IVF transfer — won't always agree exactly, and that's expected rather than a sign something is wrong. Each one narrows down the same underlying uncertainty using different information, so the conception-date and IVF methods, which start from a more precisely known point in time, are generally treated as more reliable than the last-period method when both are available.
Sources
This calculator uses Naegele's rule and standard obstetric dating conventions, the same starting point used by most clinicians. For general reading on due dates, pregnancy dating, and prenatal care, see the American College of Obstetricians and Gynecologists (ACOG) and Mayo Clinic.