GUIDE
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. A first-trimester ultrasound provides the most accurate dating, especially if your cycles are irregular.
Your due date is an estimate, not a deadline. Only about 5 percent of babies arrive on their exact due date. Most are born within a 2-week window around it — between 38 and 42 weeks. Understanding how your due date is calculated helps set realistic expectations.
Be prepared from day one
Naegele's rule, developed in the 19th century and still used today, calculates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). An equivalent shortcut is to subtract 3 months from your LMP date and add 7 days. For example, if your LMP was January 1, your estimated due date would be October 8.
This method assumes a 28-day cycle with ovulation on day 14. It also assumes that the first 2 weeks of pregnancy are pre-conception — which is why "40 weeks pregnant" really means 38 weeks since fertilization.
Naegele's rule works well for people with regular cycles close to 28 days. For shorter or longer cycles, or irregular cycles, ultrasound dating is more reliable.
Your provider will use the most accurate method available — often a combination of LMP and first-trimester ultrasound.
A first-trimester dating ultrasound is the most accurate way to establish a due date. Between weeks 6 and 12, the embryo grows at a remarkably consistent rate across all pregnancies. By measuring the crown-rump length (the distance from head to bottom), your provider can estimate gestational age with a margin of error of just 3 to 5 days.
After the first trimester, ultrasound dating becomes less precise because growth rates begin to vary between babies. A second-trimester ultrasound has a margin of error of 1 to 2 weeks, and third-trimester dating is even less accurate. This is why early prenatal care matters — your first prenatal visit is the ideal time for accurate dating.
A due date is a useful planning tool, but understanding its limitations prevents unnecessary stress.
If you have a regular 28-day cycle, Naegele's rule provides a good initial estimate. Note the first day of your last period and count forward 280 days (or 40 weeks). Many pregnancy apps and calculators do this math for you. Share this date with your provider at your first appointment.
Your dating ultrasound, usually performed between weeks 6 and 12, will measure the baby and compare size to your LMP-based estimate. If there is a discrepancy of more than 5 to 7 days, your provider will likely adjust your due date. Trust the ultrasound dating — it is more reliable than LMP in most cases.
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are 35 days, you likely ovulated around day 21, making your due date about a week later than Naegele's rule predicts. Subtract 14 from your typical cycle length to estimate your ovulation day, then count 266 days from there.
Rather than fixating on a single date, think of your due date as the center of a 4-week window. Most babies are born between 38 and 42 weeks. Full term is defined as 39 to 40 weeks and 6 days. This perspective reduces anxiety as the date approaches and passes.
Accurate dating is important for prenatal care decisions, so share any concerns about your dates with your provider.
Your due date gives you a target to plan around, even though the exact day is unpredictable. Knowing your estimated date helps you schedule prenatal appointments, plan maternity leave, and mentally prepare for your baby's arrival.
If you just found out you are pregnant, our positive test — now what? guide covers everything else you need to do in the first few days. Our week-by-week guides starting at 4 weeks pregnant will walk you through each stage. And our when to announce your pregnancy guide can help you decide when to share the news — your due date is often one of the first questions people ask.
For understanding the hormonal changes happening right now, check out our hCG levels guide to see how things are progressing behind the scenes.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
