Pregnancy Due Date Calculator

Estimate your baby's due date from the first day of your last period, the conception date or an IVF transfer date, and see how many weeks pregnant you are.

How to use the pregnancy Due Date Calculator

  1. Choose how you want to calculate: from your last period, a known conception date, or an IVF transfer date.
  2. Pick the date.
  3. If you are using your last period, enter your average cycle length. Leave it at 28 days if you are not sure.
  4. The result updates instantly as you type. There is no button to press.
  5. Use Copy result to copy the figures, or Copy link to share a link that reopens the pregnancy Due Date Calculator with the same inputs.

How it works

A pregnancy is conventionally dated from the first day of the last menstrual period (LMP), not from conception, because the LMP is a date most people know. A full-term pregnancy lasts about 280 days, or 40 weeks, from the LMP. In a typical 28-day cycle, ovulation and conception happen around day 14, which is why a baby conceived on a given day is already described as "2 weeks pregnant".

This calculator uses Naegele's rule: add 280 days to the LMP. If your cycles are longer or shorter than 28 days, ovulation likely happened later or earlier, so the due date is shifted by the difference. From a known conception date it adds 266 days (38 weeks). For IVF, the embryo's age at transfer is known exactly, so it adds 261 days for a day-5 blastocyst transfer and 263 days for a day-3 transfer.

A due date is an estimate, not a deadline. Only about 4–5% of babies are born on their due date. Most arrive in the window from 37 to 42 weeks. An early ultrasound, ideally between about 8 and 14 weeks, measures the baby directly and is the most accurate way to date a pregnancy. If it differs from your LMP date, your care provider will usually go with the scan.

Formula

Due date = LMP + 280 days + (cycle length − 28) Due date = conception + 266 days Due date = transfer date + 261 (day-5) or 263 (day-3) Gestational age = today − (due date − 280 days)

Gestational age is shown in completed weeks and days. Trimesters: first up to 13 weeks 6 days, second up to 27 weeks 6 days, third from 28 weeks.

Example

If the first day of your last period was January 10 and your cycle is 28 days, add 280 days: the estimated due date is October 17. With a 32-day cycle, ovulation probably happened about 4 days later, so the due date moves to October 21. A day-5 embryo transfer on February 1 gives a due date 261 days later, on October 20 (in a non-leap year).

Frequently asked questions

How accurate is a due date?

It is an estimate. Most babies are born between 37 and 42 weeks, and only a small percentage arrive on the exact day. An early ultrasound gives the most reliable dating.

Why is pregnancy counted from the last period?

Because the start of the last period is usually known, while the exact day of conception is not. Counting from the LMP adds about two weeks before conception.

My scan gave a different due date. Which is right?

Early ultrasound dating is generally more accurate than the LMP method, especially if your cycles are irregular. Your midwife or doctor will tell you which date they are using.

Disclaimer: This calculator provides general estimates for education only and is not medical advice, diagnosis or treatment. Formulas are based on population averages and may not fit your body, age, pregnancy or health conditions. Talk to a doctor, midwife or registered dietitian before making decisions about your health.