Due date calculator
When are you due?
Enter the first day of your last period. We use the standard 40-week estimate, adjusted for your cycle length. Your care team’s ultrasound date is the one that matters.
Your estimate
Only about 4 in 100 babies arrive on their estimated due date. A due date is a window, not an appointment.
How the due date is calculated
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. The standard “Naegele’s rule” adds 280 days — 40 weeks — to that date. If your cycle is longer or shorter than 28 days, we adjust the estimate by the difference, because ovulation usually happens about 14 days before the next period.
An early ultrasound, usually between 8 and 13 weeks, is more accurate than LMP dating, especially if your cycle is irregular or you are unsure of the date. Your hospital or midwife will use the ultrasound estimate for appointments, screening and leave planning.
What the trimesters mean
- First trimester: weeks 0–13. Early development, first appointments and often the toughest symptoms.
- Second trimester: weeks 14–27. Usually the most comfortable stretch, plus the morphology scan around 18–22 weeks.
- Third trimester: weeks 28–birth. Growth checks, birth planning and the final stretch.
If your dates change
It is normal for a due date to move after an ultrasound, particularly if your periods are irregular. Ask your care team to explain which date they are using and why. For work and leave planning, treat the date as a range and hand over earlier than you think you need to.
This calculator is general information, not medical advice. For questions about your pregnancy, contact your midwife, GP or maternity service. If you are worried about bleeding, pain, reduced movements or anything else, seek medical advice promptly.