Enter your last menstrual period date and instantly calculate your estimated due date, current pregnancy week, trimester, and days until your baby arrives.
Based on Naegele's Rule (LMP + 280 days). Confirm with your first ultrasound.
Quick answer: Your estimated due date (EDD) is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP), following Naegele's Rule — the same formula OB-GYNs use worldwide.
A pregnancy due date calculator estimates your baby's expected date of arrival — called the Estimated Due Date (EDD) — based on the first day of your last menstrual period (LMP). This is the same method used by obstetricians worldwide. The formula adds 280 days (40 weeks) to your LMP date, following Naegele's Rule.
Naegele's Rule: EDD = LMP + 280 days. This accounts for the 2-week gap between LMP and ovulation, plus 38 weeks of fetal development. Your OB-GYN uses this at your first prenatal visit and refines it with your 8–12 week ultrasound.
| Trimester | Weeks | Key Developments |
|---|---|---|
| 1st Trimester | Weeks 1–13 | All major organs form. Morning sickness peaks weeks 8–10. |
| 2nd Trimester | Weeks 14–27 | Energy returns. Baby movements begin (weeks 18–22). |
| 3rd Trimester | Weeks 28–40 | Rapid baby growth. Braxton Hicks, back pain common. |
| Week | Milestone |
|---|---|
| Week 6–8 | First prenatal visit, heartbeat detected on ultrasound |
| Week 12 | End of highest-risk period, first-trimester screening |
| Week 20 | Anatomy ultrasound — gender can be revealed |
| Week 24 | Viability milestone — baby can survive with intensive support |
| Week 28 | Third trimester begins, biweekly OB visits start |
| Week 37 | Full term — baby is ready for birth ✅ |
| Week 40 | Due date arrives! 🎉 |
It's common for your due date to shift slightly as your pregnancy progresses, and this doesn't mean anything went wrong with the original calculation. At your 8–12 week ultrasound, your doctor measures the embryo's crown-rump length, which is actually a more accurate predictor of gestational age than LMP alone — especially if you have irregular cycles or aren't certain of your exact last period date. If the ultrasound measurement differs from your LMP-based date by more than about 5–7 days in the first trimester, your provider will typically adjust your official due date to match the ultrasound. Once this adjustment happens, though, your due date is usually locked in — later ultrasounds are used to check growth, not to re-date the pregnancy, since growth rates naturally vary more in the third trimester.
It's worth setting expectations early: your due date is a statistical estimate of the midpoint of a normal range, not a deadline your body is working toward. A full-term pregnancy can end anywhere from 37 to 42 weeks and still be considered completely normal. First-time pregnancies, in particular, often go a few days past the calculated due date. Rather than counting down to one specific day, many OB-GYNs encourage thinking in terms of a "due window" spanning the last two weeks of your pregnancy.
Going past your due date is extremely common, especially with a first pregnancy, and isn't automatically cause for concern. Most providers will continue routine monitoring — sometimes with more frequent check-ins, non-stress tests, or ultrasounds to check amniotic fluid levels — through 41 weeks. Around 41 to 42 weeks, many OB-GYNs will discuss labor induction, since research shows outcomes are generally best when pregnancies don't extend significantly past 42 weeks. If you're approaching or past your due date, your care team will walk you through the specific monitoring plan and options that make sense for your individual pregnancy.
As your due date approaches, most providers recommend having your hospital bag packed by around 36 weeks, confirming your birth plan preferences with your care team, and knowing the signs that mean it's time to call your provider or head to the hospital. It's also a good time to line up support — whether that's a partner, family member, or doula — and make sure you know the fastest route to your delivery location, just in case labor moves quickly.
Many couples wait until after the first ultrasound confirms the pregnancy and gives a more reliable due date before sharing the news widely, since early miscarriage risk is highest in the first few weeks. There's no right timeline here — some share immediately with close family, others wait until the "safer" second trimester. Whatever you choose, having your due date confirmed by ultrasound first can help you share a number you feel confident about.
Medical Disclaimer: This tool is for educational purposes only. Due date estimates may vary. Always consult a qualified OB-GYN for personalized pregnancy care.