Due date calculator based on ovulation: when is my baby due?

How the due date calculator based on ovulation works

Pregnancy is officially counted from the first day of your last period (LMP), which is why a standard due date calculator adds 280 days to it. But that rule assumes you ovulated on day 14. If you know your ovulation day, from a positive LH test, a temperature shift or fertility treatment, counting from ovulation skips the guesswork.

The formula

Due date = ovulation date + 266 days. To say how far along you are, the calculator adds 14 days back on, because pregnancy weeks are still counted from an LMP: weeks pregnant = (days since ovulation + 14) ÷ 7.

• From a positive LH test: ovulation usually follows 24 to 36 hours later, so enter the day after your first positive.
• From basal body temperature: enter the day before your temperature rose and stayed up.
• From a single date of sex: enter that day as a close estimate. Conception can happen up to 5 days later if you ovulated after it, so expect a few days of uncertainty.

A worked example

You got a positive LH test on March 9, so you likely ovulated on March 10. March 10 + 266 days = December 1. That matches a last period around February 24 in a 28-day cycle. On the day you'd normally expect your period (about March 24), you'd already be counted as 4 weeks pregnant.

Due dates by ovulation date

Ovulation or conception on the 15th of each month (non-leap years). Add or subtract days from your own date to read across.

How accurate is a due date from ovulation?

More accurate than an LMP guess when you really know your ovulation day, especially with long or irregular cycles. Still, it's the middle of a range: a full-term birth is anywhere from 37 to 42 weeks, and only around 4 to 5% of babies arrive on their due date.

ACOG advises that a first-trimester ultrasound is the most accurate way to date a pregnancy that wasn't conceived through IVF. If the scan and your dates disagree by more than about a week, your provider will usually go with the scan. For IVF, the transfer date is the most exact; use the IVF due date calculator.

If you're still in the two-week wait

It's natural to plan ahead, but a test is only reliable from about the day your period is due. The DPO calculator shows what's happening each day after ovulation. The two-week wait often lands in the luteal phase mood dip too, so if you feel more anxious or tearful than usual, that's common, and it's worth being kind to yourself.