About the Due Date Calculator
Pregnancy is dated from the first day of your last period, which is roughly two weeks before you actually conceived. It is a strange convention — at four weeks pregnant the embryo is about two weeks old — but it is the one every scan, test and appointment uses, so it is worth getting used to.
If you know your conception date or had IVF, those give a tighter estimate, because they skip the guesswork about when you ovulated. All three methods are offered here.
How it works
Naegele's rule: due date = first day of the last period + 280 days. The calculator adjusts for cycle length, because a 33-day cycle ovulates about five days later than a 28-day one.
- Pick what you are counting from: last period, conception date, or embryo transfer.
- For a last period, add your average cycle length so the estimate matches your own ovulation.
- For IVF, choose whether the embryo was 3, 5 or 6 days old at transfer — the calculator counts back to conception from there.
Full term is 39 to 40 weeks and 6 days. Anything from 37 weeks counts as term; before that is preterm, and after 42 weeks is post-term, which is why most clinicians discuss induction somewhere between 41 and 42 weeks.
To translate weeks into months, use the weeks-to-months converter.
Examples
How the three methods compare for the same pregnancy:
- Last period 1 March, 28-day cycle: due 6 December.
- Last period 1 March, 35-day cycle: due 13 December — a week later, because ovulation was a week later.
- Day-5 embryo transferred 15 March: due 6 December, counted back to a conception date of 10 March.
A dating scan between 8 and 14 weeks measures the baby directly and is accurate to about five days, which is better than any calculation. If the scan disagrees with your dates by more than about a week, the clinic will usually go with the scan.
Due Date Calculator: frequently asked questions
- How accurate is a due date?
As a single day, not very — about 4% of babies are born on it. As a midpoint it is good: roughly 8 in 10 births happen within two weeks either side.
- Why is pregnancy dated from my last period?
Because it is the one date most people can name with confidence. Conception itself is rarely observed, so obstetrics standardised on the period instead and added the two weeks.
- My cycle is not 28 days — does that matter?
Yes, and most calculators ignore it. A longer cycle means later ovulation and a later due date; the adjustment here is one day per day of difference.
- Does IVF change the calculation?
It makes it more precise. The transfer date and the embryo's age fix conception exactly, so there is no assumption about ovulation at all.
- What if I am past my due date?
It is common, especially for a first baby. Most clinicians monitor from 40 weeks and offer induction between 41 and 42 weeks. Follow your own team's advice rather than a calculator.