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Only around 4 percent of babies arrive on their exact due date; most are born within a couple of weeks either side. Your dating scan at around 12 weeks gives the most accurate estimate, and midwives will use that date. This tool is a guide only, not medical advice.
What The Due Date Calculator Works Out
- Estimated due date: pregnancy is dated from the first day of your last period, not from conception, and lasts about 40 weeks (280 days) from that date. This is Naegele's rule, the convention the NHS uses. A longer or shorter cycle shifts ovulation, so the calculator adds or subtracts the difference from 28 days.
- Conception and IVF dates: from a known conception date the estimate is that date plus 266 days. IVF dating is more precise because the transfer date is exact: a day 5 blastocyst transfer adds 261 days and a day 3 transfer adds 263.
- How far along today: weeks and days since the period-equivalent start date, with the trimester. The first trimester runs to the end of week 12, the second to the end of week 26, the third to birth.
- Days to go: the count to the due date. Only around 1 in 20 babies arrives on the day itself, and anything from 37 to 42 weeks is considered normal.
- Milestones: the dates of the dating scan (about 12 weeks), the anomaly scan (about 20 weeks), the third trimester and full term, all counted from the period-equivalent date.
- Luteal phase length (advanced): the gap between ovulation and the next period, 14 days on average but anywhere from 10 to 16. If you know yours from tracking, entering it refines the ovulation and due date estimate in last-period mode.
- Dating scan (advanced): enter the scan date and the weeks and days the sonographer measured and the calculator works out the scan-based due date and how far it differs from the date-based estimate. The 12-week scan measures the baby directly and is the date your midwife will actually use.
- Estimated conception (advanced): the likely ovulation day, around two weeks after the last period in a 28-day cycle, or the fertilisation date for IVF.
- Term window (advanced): 37 weeks to 42 weeks, the range in which most babies arrive. Induction is usually offered from 41 weeks.
- Multiples (advanced): twins are usually offered planned birth from 37 weeks (36 weeks if they share a placenta) and triplets from 35 weeks, so the card shows that earlier date.
- More dates (advanced): the booking appointment, whooping cough and RSV vaccine windows, the MATB1 maternity certificate from 20 weeks, the glucose test window, the deadline for telling your employer (the 15th week before the week the baby is due), the earliest maternity leave start (11 weeks before the due date) and the induction point. This calculator is for guidance only and is not medical advice; speak to your GP or midwife about your pregnancy care.
Source and further reading: NHS: working out your due date.