How to Calculate Your Exact Due Date

Ever thought about asking Alexa (or Siri, or Google, or whatever smart assistant you use) a TTC-related question? Try this one: if today is the day you ovulate, ask what date it is 266 days from now. You'll get…

Your Baby's Due Date

Did you know it's much more accurate to calculate your estimated due date from your day of ovulation than it is from "Naegele's rule," the rule most doctors' offices default to? (Naegele's rule: add seven days to the first day of your last period, then subtract three months.) This is especially true if your cycles are irregular.

Adding 266 days to your ovulation date is more accurate because if you ovulate later than usual, your due date automatically shifts later too — it moves with your actual cycle instead of assuming everyone ovulates on day 14.

For example, if you ovulate on cycle day 30 while a standard calculation assumes ovulation on day 14, you could be in for a bit of a scare when your baby measures "2 weeks behind" on the first ultrasound — when really, nothing is wrong, the due date estimate was just off from the start. Go by "266 days from ovulation" instead, and you'll have a much more accurate timeline for when to expect your little one.

Where the 266 Number Comes From

Pregnancy length is usually described as 40 weeks (280 days) from the first day of your last period, which assumes ovulation on day 14. Counted from ovulation itself instead of your last period, that same average pregnancy is 266 days — roughly 38 weeks. Both numbers describe the same average pregnancy; they're just measured from two different starting points. Counting from ovulation just removes the guesswork about when in your cycle you actually ovulated.

Why This Matters Beyond Just Curiosity

Using the wrong starting point doesn't just shift the date on a calendar — it can lead to unnecessary worry (or false reassurance) at your early ultrasounds, since dating scans are compared against an expected size for your "official" gestational age. If your own tracking shows a later ovulation than a standard calculation would assume, it's worth mentioning to your doctor; most will adjust your due date once an early ultrasound gives a more precise measurement anyway, but your own data can help make sense of a scan that looks "off" before that happens.

If you know your ovulation date from BBT charting, an OPK, or cervical mucus tracking, you're already in a better position than most people to estimate this accurately. Our fertility calculator can also help you think through timelines by age and cycle.

So if you're ovulating today and end up pregnant, now's the time to ask your smart assistant what date it is 266 days out.