Pregnancy Due Date Calculator

What Is a Pregnancy Due Date Calculator?

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 at your first prenatal visit. The calculation adds 280 days (40 weeks) to your LMP date, following Naegele's Rule — a formula developed in the 19th century and still used worldwide.

How Doctors Calculate Your Due Date

Naegele's Rule: EDD = LMP + 280 days. Why 280 days? Because this accounts for the 2-week period between LMP and actual conception, plus 38 weeks of fetal development from fertilization. Your OB-GYN uses this calculation at your first prenatal appointment and refines it with your 8–12 week ultrasound.

What Is LMP and Why Is It Used?

LMP stands for Last Menstrual Period. Since the exact date of ovulation and conception is rarely known, doctors use the LMP as a fixed starting point. The LMP is easy to identify and consistently precedes conception by approximately 2 weeks in a 28-day cycle. This is why your "gestational age" at the moment of conception is already listed as "2 weeks" in medical records.

Pregnancy Week Breakdown

First Trimester (Weeks 1–13): All major organs form. Morning sickness peaks at weeks 8–10. Highest miscarriage risk.
Second Trimester (Weeks 14–27): The "golden trimester" — energy returns, bump shows, baby movements begin (quickening at weeks 18–22).
Third Trimester (Weeks 28–40): Rapid baby weight gain. Braxton Hicks contractions, back pain, frequent urination are common.

How Accurate Is This Calculator?

For women with regular 28-day cycles, the LMP-based due date is accurate within 1–2 weeks. Irregular cycles, PCOS, or recent hormonal contraceptive use may reduce accuracy. An ultrasound performed between 8–12 weeks is more accurate than LMP dating and may revise your EDD.

Important Pregnancy Milestones by Week

Week 8: First prenatal visit, heartbeat detected, embryo looks like a baby
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 outside womb 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!

Only 5% of babies are born on their due date — is that true?

Yes! Research shows only about 4–5% of babies arrive on their exact EDD. About 90% of babies are born between weeks 37–42. The EDD is a target, not a deadline. Labor can begin naturally anywhere from week 37 to week 42.

Can I calculate my due date from conception date instead?

Yes. If you know your conception date, add 266 days (38 weeks). Use our Conception Date Calculator to work backward from your due date to find your likely conception day.

What if my periods are irregular?

Irregular cycles make LMP-based due date calculations less reliable. An early ultrasound (6–10 weeks) that measures the embryo or fetus is the most accurate way to establish your due date when cycles are irregular.

Medical Disclaimer: This tool is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for personalized guidance.