NHS Pregnancy Calculator
Use this independent UK pregnancy calculator to estimate a due date from the first day of your last menstrual period (LMP). It follows the standard 40-week dating method and lets you adjust for a menstrual cycle that is shorter or longer than 28 days.
Pregnancy due date calculator
How the Pregnancy Due Date Calculator Works
The standard estimate starts from the first day of the last menstrual period and adds 280 days, or 40 weeks. For cycles that are not 28 days long, the estimate is shifted by the difference between your usual cycle length and 28 days. The NHS notes that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period.
What You Need
- The first day of your last menstrual period.
- Your usual menstrual-cycle length if it is not 28 days.
Due Date Formula
Estimated due date = first day of LMP + 280 days + (cycle length − 28 days).
This is an estimate, not a prediction of the exact day of birth. Your NHS maternity team may use a dating scan to provide a more reliable estimate.
Worked Example
If the first day of the last period was 1 January and the usual cycle is 28 days, the calculator adds 280 days. A 30-day cycle shifts the estimate 2 days later; a 26-day cycle shifts it 2 days earlier.
How Accurate Is a Pregnancy Due Date?
A due date is an estimate. LMP dating assumes the date is known and that cycle timing is reasonably representative. If you do not know the first day of your last period, or your dates are uncertain, the NHS advises speaking to a midwife or GP. A dating scan offered as part of antenatal care can estimate pregnancy dates more accurately.
Authoritative UK Sources
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Frequently Asked Questions
Why is pregnancy counted from the last period?
Pregnancy dating convention starts from the first day of the last menstrual period, even though conception usually happens later.
What if my cycle is not 28 days?
Adjust the cycle-length field. The calculator shifts the estimate by the difference from 28 days.
What if I do not know my LMP?
If you do not know the first day of your last period or are unsure, speak to a midwife or GP rather than relying on an LMP estimate.
Can a scan change my due date?
Yes. The NHS explains that the dating scan can provide a more accurate estimate of how many weeks pregnant you are.
Important: This is an independent informational calculator. It is not affiliated with or endorsed by the NHS and does not replace advice from your midwife, GP or maternity team.