Due Date Calculator

Frequently Asked Questions

How Is a Due Date Calculated?

A due date is calculated as 280 days from the first day of your last menstrual period, adjusted for cycle length. That’s Naegele’s rule, named for the German obstetrician who popularized it in the early 1800s. The rule assumes a 28-day cycle with ovulation on day 14. Longer cycles push the due date later. Shorter cycles pull it earlier. The calculator applies that adjustment automatically. Conception dating swaps the 280-day count for 266 days, since fertilization happens roughly two weeks after the LMP. IVF dating uses the actual embryo age at transfer, which is why it carries the smallest margin of error of any method.

How Accurate Is a Due Date?

A due date is an estimate, not an appointment. Only about 4% of babies are born on their exact due date. Roughly 60% arrive within a week either side. The rest show up earlier or later for entirely normal reasons. First-trimester ultrasound dating is accurate to within 3 to 5 days. LMP dating for regular cycles carries a wider margin, closer to 1 to 2 weeks. IVF dating is tighter still, often within a day or two, because the embryologist knows the exact age of the embryo at transfer. That wide range is why providers give you a date, not a scheduled delivery window.

What Is Naegele’s Rule?

Naegele’s rule is the standard formula for estimating a due date: take the first day of your last period, add one year, subtract three months, and add seven days. That’s the arithmetic shorthand. The longhand version is simply LMP + 280 days. It works well for women with regular 28-day cycles. For everyone else, the rule drifts. A 35-day cycle pushes ovulation about a week later, which pushes the due date a week later. Most modern calculators, including this one, adjust Naegele’s rule for cycle length so the estimate stays honest.

How Do You Calculate Due Date From IVF Transfer?

You calculate due date from IVF transfer by adding (266 − embryo age) days to the transfer date. A day-3 cleavage-stage transfer uses 263 days. A day-5 blastocyst uses 261. A day-6 blastocyst uses 260. The math is exact because the embryologist knows the precise age of the embryo at the moment of transfer. That removes the guesswork that dogs LMP dating. If you transferred a day-5 embryo on April 10, your due date lands on December 27. IVF due dates are the most reliable estimates in obstetrics, full stop.

Can My Due Date Change?

Yes, and it often does. ACOG and ISUOG both recommend redating a pregnancy when an early ultrasound disagrees with the LMP-based estimate by more than 5 days before 9 weeks, or by more than 7 days between 9 and 14 weeks. Redating isn’t a sign that anything is wrong. It usually means ovulation happened later than the standard 28-day assumption. Once redating happens, the new date becomes the official estimated due date for the rest of the pregnancy. Screening windows, growth scans, and induction discussions all shift to match.

What Is the Difference Between Due Date and Conception Date?

The due date is the estimated day of delivery, 280 days from the LMP or 266 days from conception. The conception date is the day the egg was fertilized, roughly two weeks after the LMP. Providers track pregnancy from the LMP because it’s a visible, datable event. Ovulation and fertilization happen silently, usually around day 14. That two-week gap is baked into every due date. When someone says “you’re 20 weeks pregnant,” they mean 20 weeks from the LMP, which is about 18 weeks from conception. Both dates matter, but the due date drives clinical decisions.

How Do You Calculate Due Date From Ultrasound?

You calculate due date from ultrasound by adding (280 − gestational age at scan) days to the scan date. An 8-week scan means the due date sits 224 days after the scan. An 11-week scan pushes that to 203 days. First-trimester scans are more accurate because fetal growth is uniform in those early weeks. Past 20 weeks, individual growth variation widens the margin. If you have both an early scan and an LMP-based date, the scan usually wins. If the two disagree by more than the redating threshold, your provider will switch your due date to match the scan.

What If My Cycle Is Longer Than 28 Days?

A longer cycle shifts ovulation later, which pushes the due date later. A 32-day cycle adds 4 days. A 35-day cycle adds 7. Enter your actual average cycle length in the LMP method, and the tool handles the adjustment. Clinical dating is less forgiving. If your provider uses the standard 28-day assumption without asking about your cycle, the estimated due date may be a week or more early. An early ultrasound corrects it. If you have irregular cycles, an ultrasound is the better anchor from day one.

Why Do Only 4% of Babies Arrive on Their Due Date?

Because a due date is a statistical midpoint, not a scheduled event. The 280-day count comes from a population average. Individual pregnancies vary by two weeks in either direction for entirely normal reasons. Some babies are ready earlier. Others take longer. Inductions scheduled for convenience tend to land between 39 and 41 weeks, which spreads actual births across a wide band. The 4% figure isn’t a flaw in the math. It’s a reflection of how much normal variation exists between healthy pregnancies.

What Happens If I Go Past My Due Date?

Going past your due date is common. About 40% of first-time pregnancies reach 41 weeks. Providers usually offer monitoring after 40 weeks and discuss induction between 41 and 42 weeks. Post-term is defined as 42 weeks and beyond, and the risk of stillbirth and other complications rises after that point. The calculator flags any due date that puts you past 42 weeks. If you’re already past your due date and want to know exactly how far, the gestational age tile shows your week and day.