How to Calculate Your Pregnancy Due Date: Methods, Weeks, and What to Expect

One of the first questions after a positive pregnancy test is simple: when is my baby due? Your due date becomes the anchor for everything that follows — prenatal appointments, birth preparation, work leave, and the countdown that makes it all real.

The good news is that calculating a due date is straightforward. The important thing to understand upfront is that your due date is a best estimate, not a guarantee. Only about 5% of babies are born on their exact due date — and that's completely normal.

What Is a Pregnancy Due Date?

An estimated due date (EDD) is the date on which your baby is expected to arrive, based on the standard length of a full-term pregnancy: 40 weeks, or 280 days. Pregnancies are counted from the first day of your last menstrual period (LMP), not from the date of conception — which is typically about two weeks later.

A full-term birth is considered anywhere between 37 and 42 weeks. Most healthy babies arrive in the two-week window either side of the due date.

The Standard Calculation: Naegele's Rule

The most widely used method for estimating a due date is Naegele's Rule, developed in the 19th century and still used as the baseline in obstetrics today.

Formula: EDD = First day of your last menstrual period + 280 days

Shortcut: EDD = LMP + 9 months + 7 days

This assumes a standard 28-day cycle with ovulation occurring around day 14. If your cycles are consistently longer or shorter than 28 days, your actual ovulation date — and therefore your due date — may shift by a few days.

Step-by-Step Example

Your last menstrual period started on 10 April.

Using the shortcut: 10 April + 9 months = 10 January 10 January + 7 days = 17 January

Your estimated due date is 17 January.

Verification using the 280-day method: 10 April + 280 days = 16 January

The one-day difference comes from month length variation — both results are within the standard margin of estimation. Your doctor would record approximately 16–17 January as your EDD.

What the Result Means

Your due date marks the midpoint of your delivery window, not a deadline. Healthcare providers track your pregnancy across several key stages:

| Weeks | Stage | |---|---| | Before 28 weeks | Early pregnancy | | 28–36 weeks | Late pregnancy | | 37–38 weeks | Early term | | 39–40 weeks | Full term | | 41 weeks | Late term | | 42+ weeks | Post-term (may prompt induction) |

Most doctors will not intervene simply because you pass your due date — but if you reach 41–42 weeks, your OB-GYN will discuss monitoring or induction to ensure the baby's health.

Other Methods Doctors Use

First trimester ultrasound — An ultrasound before 14 weeks measures the baby's crown-rump length (CRL) and typically provides a more accurate due date than LMP calculation alone. If there's a discrepancy of more than 7 days between the two methods, doctors generally defer to the ultrasound date.

Known conception date — If you know your exact conception date (from IVF treatment or ovulation tracking), add 266 days (38 weeks) to get your EDD.

IVF transfer date — For day-5 blastocyst transfers, add 261 days. For day-3 embryo transfers, add 263 days.

Irregular cycles — If your cycle length varies significantly from 28 days, your actual ovulation date is different from the Naegele's Rule assumption. An early ultrasound is the most reliable way to confirm your due date in this case.

Common Mistakes People Make

Confusing LMP with conception date. These are two different dates, typically about two weeks apart. Naegele's Rule always uses the first day of your last menstrual period — not when you think conception occurred.

Worrying when the ultrasound date differs. A 5–7 day difference between your LMP-based calculation and your ultrasound dating is completely expected and normal. Embryo growth has natural variation in early weeks. Your doctor will update your EDD based on the scan.

Expecting delivery on that exact day. Fewer than 1 in 20 babies arrive on their exact due date. A birth anywhere from 38–41 weeks is a healthy, normal outcome. The due date is a reference point, not a clock.

When to Expect Your Due Date to Be Revised

Your due date may be updated after your first ultrasound (typically scheduled between 8–12 weeks). If the scan shows a different gestational size than expected, your provider will adjust the EDD accordingly. After the first trimester, due dates are rarely changed even if later scans suggest a slightly different size, because growth variation increases naturally as pregnancy progresses.

Related Calculators

• Use the Pregnancy Due Date Calculator to get your EDD instantly from your last menstrual period date
• Use the Pregnancy Calculator to see your current pregnancy week and trimester, with key milestone dates
• Use the Pregnancy Week by Week Calculator to track your baby's development at each stage of your pregnancy