How to Calculate Your Due Date: Naegele's Rule Explained

One of the first things every pregnant person wants to know is: when will my baby arrive? Your estimated due date (EDD) is the single most-referenced number throughout your entire pregnancy — it determines which week you are in, which prenatal tests are recommended, when your third trimester begins, and when your provider will begin discussing delivery options if labor hasn't started. Understanding how your due date is calculated helps you make sense of the timeline and set realistic expectations about when your baby will actually arrive.

The standard method for calculating a due date is Naegele's Rule, named after the 19th-century German obstetrician Franz Karl Naegele. The formula is straightforward: take the first day of your last menstrual period (LMP), add 280 days (or 40 weeks), and that is your estimated due date. Alternatively, you can add 7 days to the LMP and subtract 3 months — the math gives the same result. For example, if your LMP began on January 1, 2026, your EDD would be October 8, 2026.

Why 280 days? Pregnancy is counted from the first day of your last period, not from conception. This is a convention established long before ultrasound existed, when the LMP was the only reliable date available. Since ovulation typically occurs around Day 14 of a 28-day cycle, actual fertilization happens about two weeks after the LMP — meaning you were not actually pregnant for the first two weeks of your 40-week pregnancy. The full gestational age from conception to birth is closer to 38 weeks, but the obstetric convention of 40 weeks from LMP is universal.

If you know your conception date rather than your LMP, you can still calculate your due date. Add 14 days to your conception date to estimate your LMP, then apply Naegele's Rule. Alternatively, simply add 266 days (38 weeks) to your conception date. Our due date calculator supports both methods — enter either your LMP or your known conception date and it will calculate your EDD, current pregnancy week, trimester, and days remaining.

Ultrasound dating is the most accurate way to confirm or adjust your due date. A first-trimester ultrasound (before 14 weeks) measures the crown-rump length of the embryo and can date the pregnancy within 5 to 7 days of accuracy. This is why your due date may be adjusted after your dating scan — if the embryo measures a week larger or smaller than expected, your provider will update the EDD accordingly. After the first trimester, ultrasound dating becomes less precise because babies grow at more variable rates.

How accurate are due dates? Only about 5 percent of babies are born on their exact due date. The majority arrive within a window of 2 weeks before or after the EDD. Full-term pregnancy is now defined as 39 to 40 weeks and 6 days, with early term being 37 to 38 weeks and 6 days, late term being 41 weeks to 41 weeks and 6 days, and post-term being 42 weeks or beyond. Most providers will discuss induction options as you approach 41 to 42 weeks to avoid risks associated with prolonged pregnancy.

The trimester breakdown based on your due date: the first trimester runs from Week 1 to Week 13 (the embryonic period of rapid development, when miscarriage risk is highest and morning sickness is most common). The second trimester runs from Week 14 to Week 27 (the 'golden trimester' — nausea typically fades, energy returns, and you begin to show). The third trimester runs from Week 28 to Week 40 and beyond (baby grows rapidly, discomfort increases, and delivery preparation begins). At Week 37 your baby is considered early term and at Week 39 fully term.

Factors that affect your actual delivery date include your baby's position, your cervix's readiness (effacement and dilation), whether this is your first or subsequent pregnancy (subsequent labors are often shorter), and medical factors such as pre-eclampsia, gestational diabetes, or fetal growth concerns that might prompt earlier delivery. First-time parents tend to deliver slightly later than the EDD on average, while second or third-time parents often deliver slightly earlier.

To get your due date instantly, use our free due date calculator — enter your last menstrual period date or conception date and it will show your EDD, current pregnancy week and days, trimester, baby size comparison, and a trimester milestone timeline. No signup required.