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Pregnancy Due Date Calculator — From Last Period or IVF Transfer

Your due date is one of the first numbers you'll want to know. Here's how it's actually calculated — and why the method depends on how you conceived.

How a due date is estimated

A pregnancy is counted as lasting about 40 weeks from the start of your last menstrual period — not from the day you conceived, which most people can't pinpoint exactly. That 40-week standard is the basis of nearly every due-date calculation.

The most common method: last menstrual period (LMP)

If you know the first day of your last period, the classic formula is called Naegele's rule: take that date, add 280 days (40 weeks). That's your estimated due date. It assumes a 28-day cycle and ovulation around day 14, so it's an estimate — but it's the standard starting point.

If you conceived via IVF: use the transfer date

With IVF, the dates are known precisely, so the calculation is more accurate — but it depends on the age of the embryo at transfer:

• Day-5 embryo (blastocyst): due date = transfer date + 261 days.
• Day-3 embryo: due date = transfer date + 263 days.

Your clinic will give you the exact date, but it's useful to understand where the number comes from — and to check it yourself.

Why your due date is only an estimate

Only about 4–5% of babies arrive on their actual due date. A "full-term" birth can fall anywhere from 37 to 42 weeks, so it's better to think of it as a due window rather than a deadline. The due date matters most for scheduling care and tracking week-by-week progress — not for predicting the exact day.

A quick summary

• LMP method: first day of last period + 280 days.
• IVF (day-5 embryo): transfer date + 261 days.
• IVF (day-3 embryo): transfer date + 263 days.
• Reality check: most babies arrive within a 37–42 week window, not on the exact date.

Whenever the date, congratulations — and check with your midwife or doctor for anything specific to your pregnancy.