Calculate your estimated due date from your last menstrual period or conception date
Knowing your estimated due date (EDD) is one of the first things you will want to establish after a positive pregnancy test. Your due date helps you and your healthcare team plan antenatal care, monitor fetal development, and prepare for birth. This calculator uses Naegele's rule, the same method used by GPs and midwives as an initial estimate. Named after Franz Karl Naegele, this 19th-century formula calculates the due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a 28-day cycle with ovulation occurring on day 14. You can calculate from either your LMP date or your estimated conception date. If you know the approximate date of conception, the calculator subtracts 14 days to determine the equivalent LMP date and then applies Naegele's rule as normal. Both methods produce the same result when the dates are 14 days apart.
To calculate your estimated due date: 1. Select your calculation method. Choose "Last Menstrual Period (LMP)" if you know the first day of your last period, or "Conception Date" if you know the approximate date of conception. 2. Select the day, month, and year of either your LMP or conception date using the dropdown menus. 3. Review your results. The calculator displays your estimated due date, your current gestational age (in weeks and days), and which trimester you are in. 4. Remember that this is an estimate. Only about 5% of babies arrive on their exact due date. Your NHS dating scan between 8 and 14 weeks will provide a more accurate estimate based on the size of the fetus, especially if your cycle is irregular or longer/shorter than 28 days.
This calculator uses Naegele's rule: **Estimated Due Date = LMP + 280 days (40 weeks)** Pregnancy is conventionally measured from the first day of the last menstrual period, even though conception typically occurs about two weeks later (at ovulation). This means that at the time of a missed period, you are already considered approximately 4 weeks pregnant. If you enter a conception date instead of LMP, the calculator subtracts 14 days from your conception date to estimate the equivalent LMP, then applies the standard 280-day calculation. This produces the same due date as entering an LMP that is 14 days before the conception date. Trimesters are defined as: - First trimester: Weeks 0-12 (up to 13 weeks) - Second trimester: Weeks 13-26 (up to 27 weeks) - Third trimester: Weeks 27-40+ Naegele's rule assumes a regular 28-day menstrual cycle. If your cycle is consistently longer or shorter, your actual ovulation date may differ, which can shift the due date by several days.
This calculator provides an estimate based on Naegele's rule and is not a substitute for professional medical advice. Your NHS dating scan (offered between 8 and 14 weeks) uses ultrasound measurements to provide a more personalised estimate, especially if your cycle length varies. If you have any concerns about your pregnancy or due date, contact your midwife or GP. The NHS Pregnancy and Baby guide at nhs.uk provides comprehensive information about antenatal care and what to expect at each stage.