Due Date Calculator
Estimate your baby's due date from your last menstrual period or conception date, using Naegele's rule, with key pregnancy milestones.
This uses Naegele's rule: 280 days from the first day of your last period, which assumes a regular 28-day cycle with ovulation on day 14. The small print of the rule is that it adds one year, subtracts three months and adds seven days — the same 280 days counted from the LMP. If your cycle is longer or shorter, or you conceived through IVF or with a known ovulation date, the dating scan is the more reliable estimate and will usually be used by your midwife or doctor instead.
Frequently asked questions
How is a due date calculated?
By Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. If conception happened by IVF or a known date, the due date is 266 days from conception.
How accurate is a due date?
It is an estimate. Most babies arrive within two weeks either side of the due date, and only a small proportion are born exactly on it. A dating scan is more accurate than dates alone.
Should the due date be adjusted after a scan?
Usually yes for early scans. A first-trimester dating scan is considered more accurate than LMP dates, and healthcare teams will often revise the estimated due date accordingly.
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