Estimate your baby’s due date from the first day of your last period and your usual menstrual-cycle length.
Enter your dates
Use the first day—not the last day—of your most recent menstrual period. Enter dates in UK format: DD/MM/YYYY.
Estimated pregnancy timeline
Medical note: This calculator gives an estimate and does not replace advice from a midwife, GP or maternity unit. If you do not know the first day of your last period, your cycles are irregular, or you have questions about symptoms or pregnancy dates, contact a healthcare professional.
How the pregnancy due date calculator works
The NHS estimates a due date from the first day of the last menstrual period. A 28-day-cycle estimate adds 280 days, or 40 weeks. This calculator adjusts that date by the difference between your usual cycle length and 28 days.
| Input | How it is used |
|---|---|
| First day of the last period | Starting point for pregnancy dating |
| Cycle length | Days above or below 28 are added to or subtracted from the estimate |
| Estimated due date | Last-period date + 280 days + cycle-length adjustment |
Why a due date is only an estimate
A due date marks the estimated point at which pregnancy reaches 40 weeks. The NHS explains that pregnancy normally lasts from 37 to 42 weeks from the first day of the last period, and it is common to give birth before or after the estimated date.
Ovulation does not always happen at the same point in every cycle. The last-period method can therefore be less reliable when cycles are irregular, the date is uncertain, hormonal contraception was recently stopped or bleeding was not a normal period.
Dating scans and the estimated due date
In England, ultrasound scans are normally offered at 11 to 14 weeks and again between 18 and 21 weeks. At the first scan, the sonographer uses the baby’s measurements to estimate the due date. That scan estimate may replace the date calculated from the last period.
The timeline shown by this calculator is approximate. Appointment availability and local maternity pathways vary, so follow the dates provided by your midwife or hospital.
What if I do not know my last-period date?
Do not guess if you are unsure. The NHS advises speaking to a midwife or GP. A dating scan can provide a more useful clinical estimate based on measurements.
Understanding the 37-to-42-week window
The estimated due date is the 40-week point, not a guaranteed birth date. The calculator also shows estimated 37-week and 42-week dates to illustrate the broad NHS pregnancy-duration window. These dates must not be used to decide when to seek care.
When to contact a healthcare professional
Contact your midwife, GP, NHS 111 or maternity unit if you need advice about your pregnancy. Seek urgent medical help for severe symptoms, heavy bleeding, intense pain or any situation your maternity team has told you requires immediate assessment.
NHS guidance: Pregnancy due date calculator, Ultrasound scans in pregnancy and Pregnancy and due dates.
Frequently asked questions
How is a pregnancy due date calculated?
For a 28-day cycle, 280 days are added to the first day of the last period. This calculator also adjusts for the entered cycle length.
Is the estimated due date exact?
No. It is an estimate of the 40-week point. Pregnancy commonly lasts between 37 and 42 weeks.
Can a dating scan change my due date?
Yes. The sonographer estimates the due date from the baby’s measurements, and your maternity team may use that scan-based date.
What cycle length should I enter?
Enter your usual number of days from the first day of one period to the first day of the next. If cycles are irregular or you are unsure, speak to a midwife or GP.
What if I do not know the first day of my last period?
The NHS advises speaking to a midwife or GP rather than relying on a guessed date.