How a due date is calculated
Doctors and midwives usually date a pregnancy from the first day of your last menstrual period (LMP), not from conception, because most people know that date and conception is hard to pin down. A full-term pregnancy is counted as 40 weeks (280 days) from the LMP. This method is known as Naegele's rule.
due date = first day of last period + 280 days
(add or subtract the days your cycle is longer or shorter than 28)
The 280 days assume a 28-day cycle, with ovulation around day 14. If your cycle is longer, you likely ovulated later, so the due date moves later by the same number of days. A 30-day cycle adds 2 days, and a 26-day cycle takes 2 days away.
If you know the conception date, for example from IVF or ovulation tracking, the due date is 266 days (38 weeks) after conception.
Worked example
Your last period started on 1 March 2026 and your cycle is 28 days.
1 March + 280 days = 6 December 2026. Conception most likely happened around 15 March.
With a 30-day cycle, the due date moves 2 days later, to 8 December 2026.
Pregnancy trimesters
| Trimester | Weeks | Roughly |
|---|---|---|
| First | Week 1 to the end of week 13 | Months 1–3 |
| Second | Week 14 to the end of week 27 | Months 4–6 |
| Third | Week 28 until birth | Months 7–9 |
Because dating starts at your last period, the first two "weeks" of pregnancy happen before conception. That's why you may be told you're 4 weeks pregnant only two weeks after conceiving.
How accurate is a due date?
A due date is an estimate. Only about 1 in 25 babies (around 4%) is born on the exact day. Most are born within two weeks either side: a birth from 37 to 42 weeks is considered term. An early ultrasound scan, especially in the first trimester, measures the baby and is often used to confirm or adjust the date, particularly if your cycles are irregular.
Important: this calculator is for general information only and isn't medical advice. Your doctor or midwife will confirm your due date, usually with an ultrasound. Contact them about any concerns during pregnancy.
Frequently asked questions
What if I don't know the date of my last period?
An ultrasound can estimate how far along you are by measuring the baby. In early pregnancy this is accurate to within about a week, so your care provider will usually book a dating scan.
My cycles are irregular. Will this work?
It will be less reliable, because it's hard to know when you ovulated. Use your average cycle length for a rough idea, and rely on an ultrasound for a more accurate date.
Why is my scan date different from this calculator?
Babies grow at slightly different rates and ovulation timing varies. If an early scan differs from the period-based date by more than about a week, providers usually go with the scan.
Does this work for IVF pregnancies?
Choose "Conception date" and enter the egg retrieval or insemination date. For a frozen embryo transfer, your clinic will give an exact due date based on the embryo's age, so use theirs.
Last reviewed November 2026.