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Due Date Calculator

Enter the first day of your last period and your usual cycle length. You get an estimated due date, how far along you are on a date you choose, and when each trimester starts.

Your numbers

Result

Estimate for information only, not professional advice. Last reviewed .

How we calculate it

Naegele’s rule adds 280 days, which is 40 weeks, to the first day of the last period. This is the same as adding one year, taking away three months and adding seven days. If your cycle is not 28 days, the estimate moves by the difference: a 32 day cycle adds 4 days and a 26 day cycle takes away 2.

For a last period starting on 10 January 2026 and a 28 day cycle, the due date is 17 October 2026. The second trimester starts in week 14 (day 98) and the third in week 28 (day 196), counted from the same start.

If you know the conception date, the due date is 266 days (38 weeks) after it, which is the same as counting 14 days back to a last period. For IVF the embryo transfer date stands in for conception: a day 3 embryo adds 263 days and a day 5 embryo (blastocyst) adds 261 days, because the embryo is already 3 or 5 days old at transfer.

Assumptions and limits

  • This is an estimate for general information. It is not medical advice and cannot replace a conversation with a doctor or dietitian. Only about 5% of babies are born on their due date, and most arrive within a couple of weeks either side.
  • If an early ultrasound gave a different date, your doctor or midwife’s dating takes priority over this calculator.
  • The calculator works for dates up to 320 days after the start of the pregnancy dating. After the due date it shows a note.
  • For IVF and other assisted pregnancies your clinic dates the pregnancy from the transfer, so use its due date if it differs from this estimate.
  • Sources: ACOG Committee Opinion 700 (Methods for Estimating the Due Date) and Committee Opinion 579; the conception rule (due date = conception + 266 days) and the IVF rule (transfer date + 266 days − embryo age: 263 for day 3, 261 for day 5) as stated by Perinatology.com and fertility clinics (Illume Fertility, Dallas IVF).

Frequently asked questions

How is a due date calculated?

By adding 40 weeks to the first day of the last menstrual period. Doctors and midwives may adjust it using an ultrasound scan.

How likely is it that I give birth on my due date?

Not very. Only about 5% of babies are born on the estimated date. Most are born in the weeks before or after it.

How is the due date worked out from conception or an IVF transfer?

Add 266 days to the conception date. For IVF add 263 days to a day 3 embryo transfer date and 261 days to a day 5 transfer date. All three give the same due date as a last period 14, 17 or 19 days earlier.

Does my cycle length change the date?

It can. The rule assumes ovulation about 14 days after the period starts in a 28 day cycle. If your cycle is longer or shorter, ovulation is later or earlier, so the date moves by the same number of days.

Related calculators

Worked example (a fixed example using the starting numbers, it does not change as you type)

Inputs: Date the pregnancy from = First day of last period; First day of your last period = January 10, 2026; Usual cycle length (days, 21 to 45) = 28; Date to check = March 21, 2026.

  • Estimated due date Saturday, October 17, 2026
  • Weeks pregnant on that date 10 weeks 0 days
  • Days to go 210
  • Second trimester starts Saturday, April 18, 2026
  • Third trimester starts Saturday, July 25, 2026

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Last reviewed: . Formulas and assumptions are described on our methodology page. For information only; not professional advice.