Your Due Date Is Probably Wrong. Here's Why That Matters.

There are two main methods for calculating your due date, and they can give you different answers. **Method 1: Last Menstrual Period (LMP)** The standard calculation is called Naegele's Rule: take the first day of your last menstrual period, add 280 days (40 weeks). This assumes you ovulated on day 14 of a 28-day cycle. The problem? Most women don't have 28-day cycles, and most don't ovulate on day 14. A study in the journal Nature found that only 13% of women ovulate on day 14. The average is closer to day 16, and the normal range is day 10 to day 23. If you ovulated on day 20 instead of day 14, your due date is off by 6 days. That might not sound like much, but at 40 weeks, 6 days matters. **Method 2: First Trimester Ultrasound** A first trimester ultrasound (between 8-12 weeks) measures the crown-rump length of the fetus and compares it to established growth charts. This is more accurate than LMP dating because it measures actual development rather than estimating from your cycle. Ultrasound dating at 8-12 weeks is considered accurate to within 5-7 days. That's much better than LMP, which can be off by 2-3 weeks depending on your cycle length and ovulation timing. **When they disagree:** If your LMP says February 15 but your ultrasound says February 22, most providers will use the ultrasound date if the difference is more than 7 days in the first trimester. This is why your due date might "change" after your first scan. The date didn't actually change — the measurement got more precise. (Curious about your own calculation? Our [due date calculator](/en/calculator/due-date-calculator) lets you input either your LMP or conception date and see the estimated range.)

**Think of your due date as a due month, not a due day.** This is the single most helpful reframe I offer patients. Instead of "the baby is due March 12," think "the baby will likely arrive between March 1 and March 26." A full-term pregnancy is anywhere from 37 to 42 weeks. That's a five-week window. Your due date is the middle of that window, not the end of a countdown timer. **Ask which method your provider used.** This matters more than most people realize. If your due date was calculated from LMP alone and you have irregular cycles, the date could be off by 2-3 weeks. If it was confirmed by first trimester ultrasound, it's much more reliable. Don't be afraid to ask: "Was my due date based on my period or the ultrasound?" And if it was based on LMP and you have cycles longer than 30 days, mention that. **Don't panic at 40 weeks + 1 day.** First-time mothers, on average, deliver at 41 weeks + 1 day. Not 40 weeks. 41 weeks and 1 day. If this is your first baby, going past your due date isn't just normal — it's expected. The average second baby arrives at 40 weeks + 3 days. Your body isn't "late." The date was early. **Know when post-dates actually becomes concerning.** Most providers start monitoring more closely at 41 weeks and discuss induction at 41.5-42 weeks. The risks of going past 42 weeks include decreased amniotic fluid, placental aging, and increased stillbirth risk (though the absolute risk is still very low — about 1 in 1,000 at 42 weeks vs 1 in 1,500 at 40 weeks). This is a conversation to have with your provider, not a reason to panic. **Use our tools to track your actual window.** Our [due date calculator](/en/calculator/due-date-calculator) gives you a range, not just a date. And our [pregnancy calculator](/en/calculator/pregnancy-calculator) tracks week-by-week development. Seeing the full window helps reduce the "why isn't the baby here yet" anxiety that builds when you fixate on one specific date.

Mistake one: treating the due date as a deadline. I've seen patients induce anxiety, schedule inductions they didn't medically need, or make frantic arrangements because "the baby was supposed to be here Tuesday." The due date is an estimate, not an expiration date. Babies don't have calendars. Mistake two: changing your due date based on later ultrasounds. First trimester ultrasatures are the most accurate for dating. Second and third trimester ultrasounds measure size, not age. A 32-week ultrasound might say "measuring 34 weeks" — that means the baby is measuring large for gestational age, not that your due date was wrong. But I've seen patients and even some providers adjust dates based on later scans, which leads to incorrect timing of important decisions about induction and delivery. Mistake three: comparing yourself to everyone else's timeline. Social media is full of "I had my baby at 38 weeks!" posts. That's great for them. It tells you nothing about when your baby will arrive. Every pregnancy is different, and comparing your timeline to someone else's is a recipe for unnecessary anxiety. Your body and your baby are on their own schedule. The best thing you can do with your due date is treat it as a helpful approximation, prepare for a range, and trust that your provider will let you know if anything actually needs attention. Most of the time, everything is fine — just not on the exact date the calendar predicted.