Due Date Calculator

What Is the Due Date Calculator?

A pregnancy due date, also known as the estimated date of delivery (EDD), is the date on which a pregnancy is expected to reach 40 weeks from the first day of the last menstrual period. It is an estimate rather than a fixed date. According to the American College of Obstetricians and Gynecologists, only around 5 percent of births occur exactly on the calculated due date, but the figure is essential for planning antenatal care, assessing fetal growth, and making clinical decisions throughout pregnancy.

This calculator uses Naegele's Rule, the standard method for estimating the due date from the last menstrual period. The NHS also recommends dating ultrasound scans, typically carried out between 10 and 14 weeks, as the most accurate way to confirm the EDD, particularly if periods are irregular or the LMP date is uncertain. With that in mind, treat the result here as a starting estimate to bring to your first antenatal appointment.

How to Use the Due Date Calculator

1. Enter the date of the first day of your last menstrual period (LMP).
2. Enter your average cycle length. The default is 28 days, but you can adjust this if your cycles are longer or shorter.
3. The calculator works out your estimated due date and displays how many weeks pregnant you currently are.
4. It also shows your trimester breakdown and approximate dates for key milestones.
5. If you have had a dating ultrasound, your midwife or doctor will use that result to confirm or adjust the EDD.

The Formula: Naegele's Rule

Naegele's Rule is the standard clinical method for calculating a due date from the last menstrual period. It was set out by German obstetrician Franz Karl Naegele in the early 19th century and remains the foundation of most due date calculations today.

EDD = First day of LMP + 280 days (40 weeks)

For cycles that differ from the standard 28 days, an adjustment is applied. If your cycle is longer than 28 days, the extra days are added. If it is shorter, they are subtracted. For example, if your cycle is 35 days, 7 days are added to the standard calculation. This makes the formula more accurate for people whose cycles are not the assumed average length.

Worked example: If your LMP began on 1 January, your estimated due date using Naegele's Rule is 8 October of the same year, assuming a 28-day cycle.

Pregnancy Trimesters

Key Considerations

Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14. In practice, cycle lengths vary considerably and ovulation does not always occur on day 14, even in regular cycles. Given that, the formula's accuracy declines for people with irregular periods or conditions such as polycystic ovary syndrome that affect cycle regularity.

Early ultrasound dating, carried out between 10 and 14 weeks, is more accurate than LMP-based calculation in most cases, as it directly measures fetal size rather than working back from the LMP. ACOG recommends that when there is a discrepancy of more than a week between LMP-based dating and ultrasound dating in the first trimester, the ultrasound date should be used. That said, even ultrasound dating carries a margin of error of several days.

It is also worth noting that a normal full-term pregnancy can result in birth anywhere between 37 and 42 weeks. A birth occurring before 37 weeks is considered preterm. On top of that, first pregnancies tend to go slightly longer on average than subsequent pregnancies, though the difference is usually small.

What to Do With Your Result

Your estimated due date is the starting point for your antenatal care timeline. Most maternity services will book your initial appointment and dating scan shortly after you confirm your pregnancy. Bring this result to that appointment so your midwife or doctor can compare it against the ultrasound measurement. From there, your care team will use the confirmed EDD to schedule all subsequent antenatal checks, screening tests, and growth scans.

Conclusion

The due date calculator gives you a reliable first estimate of your expected delivery date based on your last menstrual period and cycle length. It is a useful planning tool for the early weeks of pregnancy, giving you a sense of your current gestational age and when key antenatal appointments are likely to fall. That said, it is always worth confirming the date with a healthcare provider at your first appointment, where a dating ultrasound will give you the most accurate figure available.