Due Date Calculator

About the Due Date Calculator

The Due Date Calculator estimates your Expected Delivery Date (EDD) using Naegele's rule and multiple calculation methods. Knowing your due date helps you and your healthcare team plan prenatal care, schedule key ultrasounds, and prepare for delivery.

The most common method uses your last menstrual period (LMP): add 280 days (40 weeks) to the first day of your last period. If you know your conception date, the calculator adds 266 days. For IVF pregnancies, it uses the embryo transfer date plus 261 days (Day 3 transfer) or 259 days (Day 5 blastocyst transfer).

Only about 5% of babies are born on their exact due date, but 80% are born within two weeks of the EDD. Your healthcare provider may adjust the due date based on first-trimester ultrasound measurements.

When This Page Helps

An estimated due date is a practical anchor for prenatal planning. It helps frame the timing of common screening windows, anatomy scans, glucose testing, and later-term monitoring, while also providing context for gestational-age discussions.

How to Use the Inputs

1. Select your calculation method: LMP, Conception Date, or IVF Transfer.
2. Enter the relevant date.
3. For IVF, select Day 3 or Day 5 transfer.
4. View your estimated due date.
5. See your current gestational age and trimester.
6. Review the trimester timeline and key milestones.
7. Discuss the result with your obstetrician or midwife.

Example Calculation

Tips & Best Practices

• First-trimester ultrasound (6–12 weeks) dating is accurate to ±5 days and should be used to confirm or adjust the LMP-based date.
• LMP dating assumes a 28-day cycle with ovulation on day 14 — adjust for longer or shorter cycles.
• For IVF pregnancies, the transfer date method is the most accurate since ovulation timing is precisely controlled.
• Only 5% of babies arrive on the due date; the normal delivery window is 37–42 weeks.
• Post-term pregnancies (beyond 42 weeks) carry increased risk and typically prompt induction discussions.
• Keep your LMP date recorded — it's one of the first questions asked at every prenatal visit.

History of Naegele's Rule

Franz Karl Naegele, a German obstetrician, published his rule in 1812: add one year, subtract three months, and add seven days to the LMP. This is equivalent to adding 280 days and has been the standard dating method for over two centuries despite its well-known limitations.

Trimester Overview

The first trimester (weeks 1–13) is the period of organogenesis when major structures form. The second trimester (weeks 14–27) involves rapid growth and organ maturation — this is when most women feel best. The third trimester (weeks 28–40) focuses on fetal weight gain and lung maturity, with the fetus doubling in weight during the final 8 weeks.

Modern Due Date Estimation

Research has shown that the average pregnancy lasts 268 days from ovulation — slightly shorter than Naegele's 280 days from LMP. First-time mothers tend to deliver slightly later than multiparous women. Machine learning models incorporating ultrasound biometry, maternal characteristics, and biomarkers are being studied to provide more personalized due date predictions.

Frequently Asked Questions

• How accurate is Naegele's rule?

  Naegele's rule assumes a 28-day cycle and ovulation on day 14. It is reasonably accurate for women with regular 28-day cycles but can be off by one to two weeks for those with irregular or longer cycles. First-trimester ultrasound is considered the gold standard for pregnancy dating.

• What if my ultrasound date differs from my LMP date?

  ACOG guidelines recommend adjusting the due date when the ultrasound-based estimate differs by more than 5 days (before 9 weeks), 7 days (9–15 weeks), or 10 days (16–21 weeks). In these cases, the ultrasound date takes precedence.

• What is the difference between gestational age and fetal age?

  Gestational age counts from the first day of the LMP and is the standard clinical measurement (40 weeks total). Fetal age (embryonic age) counts from conception and is about 2 weeks shorter (38 weeks). Healthcare providers use gestational age for all clinical decisions.

• Does the due date change during pregnancy?

  The due date may be adjusted once based on a first-trimester ultrasound. After that, it should not be changed because later ultrasound measurements have increasing variability. ACOG recommends establishing a final EDD by the first prenatal visit.

• What happens if I go past my due date?

  Pregnancies beyond 41 weeks are considered late-term, and beyond 42 weeks are post-term. Post-term pregnancies carry increased risks including oligohydramnios, macrosomia, and stillbirth. Most providers discuss induction between 39 and 41 weeks based on individual risk factors.

• Can I calculate my due date from a positive pregnancy test?

  Not precisely. A positive home test usually occurs around 4 weeks gestational age (2 weeks after conception), but the exact detection day varies. For the most accurate dating, you need either your LMP date, conception date, or a dating ultrasound.