Due Date Calculator

About the Due Date Calculator

A due date calculator estimates the Estimated Due Date (EDD) of a pregnancy from the first day of the last menstrual period (LMP) using Naegele's Rule — the standard clinical method used by obstetricians and midwives worldwide. The EDD is 280 days (40 weeks) from the first day of the last period, based on the assumption of a 28-day menstrual cycle with ovulation on day 14. For cycles longer or shorter than 28 days, the EDD is adjusted accordingly. Our calculator also adjusts automatically for your cycle length, shows your current gestational week and trimester, calculates the days remaining until the due date, and lists the major pregnancy milestones with their expected dates — from the week when fetal heartbeat becomes detectable (week 6) through viability milestones, anatomy scans, and the full-term window. The due date is one of the most important pieces of information in obstetric care, used to time prenatal testing, plan delivery, and assess fetal growth. Understanding that only about 5% of births occur on the exact due date — and that normal delivery spans weeks 37-42 — helps set appropriate expectations. Relevant for pregnant women and their healthcare providers in the USA, UK, Canada, Australia, and worldwide.

How It Works

Naegele's Rule: EDD = LMP + 280 days. For non-standard cycles: EDD = LMP + 280 + (cycle length − 28) days. This adjusts ovulation timing for cycles that differ from the standard 28-day assumption. Example: LMP = January 1, 28-day cycle. EDD = January 1 + 280 days = October 8. For a 35-day cycle: add 7 extra days → EDD = October 15. Conception date = LMP + (cycle length − 14) days. For 28-day cycle: LMP + 14 days. Current gestational week = days since LMP / 7. Trimesters: 1st = weeks 1-12; 2nd = weeks 13-26; 3rd = weeks 27-40. Key milestones: Week 6 heartbeat detectable by transvaginal ultrasound; Week 12-13 nuchal translucency scan; Week 20 anatomy ultrasound; Week 24 viability threshold; Week 37 early term begins; Week 39-40 full term optimal delivery window.

Tips & Best Practices

• ✓First trimester ultrasound (weeks 6-12) is more accurate than LMP dating — it measures fetal crown-rump length (CRL) with ±3-5 day accuracy. If ultrasound dating differs from LMP dating by more than 5-7 days, most providers adjust the EDD to the ultrasound date.
• ✓Post-term pregnancy (≥ 42 weeks, 294 days) carries increased risk of placental insufficiency and stillbirth. Most providers recommend induction of labor by 41-42 weeks. Knowing your accurate due date is essential for timing induction discussions appropriately.
• ✓UK NHS dating: the NHS offers a booking scan at 8-14 weeks (ideally 10-14 weeks) for EDD confirmation by CRL measurement. This scan is considered definitive for dating. The 20-week anomaly scan confirms gestational age and checks fetal anatomy.
• ✓Multiple pregnancy due dates: twins are typically delivered at 36-37 weeks (electively); triplets at 34-35 weeks. The EDD calculated here applies to singleton pregnancies. For multiples, discuss timing with your maternal-fetal medicine specialist.

Who Uses This Calculator

Newly pregnant women calculating their due date and current gestational week. Healthcare providers confirming LMP-based dating before ultrasound confirmation. Pregnant women planning leave and baby-related decisions around the expected delivery window. Anyone who wants to see upcoming pregnancy milestones and their expected dates on a single timeline.