How to Calculate Your Due Date Correctly

Finding out you are pregnant is a moment that immediately leads to one pressing question: when is the baby due? A due date gives you a timeline for antenatal appointments, plans for time off work, and simply something to look forward to. This guide explains how due dates are calculated and what affects their accuracy.

The Two Main Methods

There are two widely used approaches to calculating a pregnancy due date. Both rely on estimating when conception occurred, just from different starting points.

Using Your Last Menstrual Period (LMP)

The most common method is Naegele's rule, developed in the 19th century and still in routine use today. The formula is simple:

Due date = First day of last menstrual period + 280 days

280 days equals 40 weeks — the standard length of a full-term pregnancy counted from the LMP. The reason the calculation starts from the LMP rather than conception is that the LMP is a known, recordable date, while conception is often estimated.

This method assumes ovulation (and therefore conception) occurs on day 14 of a 28-day cycle. If your cycle is longer — say, 35 days — ovulation typically occurs around day 21, which means conception happened approximately a week later than the 28-day model assumes. Many online calculators, including ours, allow you to adjust for cycle length.

Using Your Conception Date

If you know the approximate date of conception — perhaps because you tracked ovulation or used assisted reproduction — you can calculate your due date directly:

Due date = Conception date + 266 days

266 days (38 weeks) is used because conception typically occurs around 14 days after the LMP in a standard cycle. Adding 266 days to a conception date gives approximately the same result as adding 280 days to the LMP date.

How Accurate Are These Calculations?

Both methods are estimates. In practice, only around 5% of babies are born on their exact calculated due date. Most births occur within a window of two weeks before or after the due date, and this is considered completely normal.

Several factors limit the accuracy of date-based calculations:

• Cycle variation: Ovulation does not always occur on day 14, even in women with regular cycles.
• Memory of the LMP date: Stress, travel or illness can make the LMP date uncertain.
• Conception timing: Sperm can survive for several days after intercourse, so the exact date of fertilisation is difficult to pinpoint.

When Your Doctor May Calculate Differently

At your first ultrasound scan (usually called a dating scan, performed between 11 and 14 weeks), the sonographer will measure the baby — typically the crown-rump length — and use this to estimate gestational age. If the ultrasound-derived estimate differs by more than 7–10 days from your LMP-based date, your healthcare provider may revise the due date accordingly.

Ultrasound dating in the first trimester is generally considered more accurate than LMP-based calculations because it directly measures fetal growth rather than relying on recalled dates. A revised due date is not a sign that anything is wrong — it simply reflects more precise information.

Practical Tips for Tracking

• Record the exact date of your last menstrual period as soon as you discover you are pregnant, if you know it.
• Note your typical cycle length — this helps both your calculator and your healthcare provider.
• Attend your dating scan, as this is the most reliable way to confirm your due date.
• Remember that the due date is a midpoint estimate. Birth is equally normal up to two weeks before or after this date.

This guide is for general information only. Always discuss your due date and pregnancy timeline with your midwife or doctor.