About the Pregnancy Due Date Calculator

This Pregnancy Due Date Calculator estimates your baby’s estimated due date (EDD) using Naegele’s Rule, the standard method recommended by the American College of Obstetricians and Gynecologists (ACOG Committee Opinion #700, reaffirmed 2020). The rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP) and adjusts for cycle lengths that differ from 28 days.

Only about 5% of babies arrive on their exact due date. The EDD is the midpoint of a 4-week window: delivery between 37 weeks 0 days (early term) and 41 weeks 6 days (late term) is normal; births before 37 weeks are preterm; inductions are typically offered at 41 weeks.

Naegele’s Rule is most accurate when the menstrual cycle is regular and the LMP is confidently known. If cycle length varies or conception was assisted (IVF, IUI), first-trimester ultrasound (CRL at 7-13 weeks) gives a more accurate EDD, with a margin of ±5-7 days. ACOG recommends revising the LMP-based EDD when ultrasound differs by more than 5-7 days in the first trimester.

How It Works

Naegele’s Rule (named after 19th-century German obstetrician Franz Naegele):

Standard 28-day cycle:  EDD = LMP + 280 days (40 weeks)
                        = LMP + 9 months + 7 days
Adjusted cycle length:  EDD = LMP + 280 + (cycle_length - 28) days

The 280-day figure derives from 266 days from conception to birth plus 14 days from LMP to ovulation. For cycles shorter or longer than 28 days, ovulation occurs earlier or later than day 14, so the EDD shifts by the same number of days.

Trimester milestones (ACOG):

1st trimester: 0 - 13 weeks 6 days
2nd trimester: 14 - 27 weeks 6 days
3rd trimester: 28 weeks - delivery
Full term: 39 weeks 0 days - 40 weeks 6 days
Post-term: ≥42 weeks 0 days (typically offered induction)

Worked Examples

LMP = January 1, 2025, cycle = 28 days:

EDD = Jan 1 + 280 days = October 8, 2025
Conception estimate = Jan 1 + 14 = January 15, 2025
At Feb 5, 2025: gestational age = 5w 0d (1st trimester)

LMP = March 15, 2025, cycle = 35 days:

Adjustment = +7 days
EDD = Mar 15 + 287 days = December 27, 2025
Conception estimate = Mar 15 + 21 = April 5, 2025

The 7-day shift reflects later ovulation in a 35-day cycle. Standard Naegele would predict December 20; the adjusted rule moves it to December 27.

For IVF pregnancies, EDD is calculated from the embryo transfer date (+266 days for fresh 3-day, +261 for 5-day blastocyst). Ultrasound CRL at 7-13 weeks remains the most accurate dating method.

When to Use This Tool

Initial pregnancy dating at home before the first prenatal visit (typically 6-8 weeks).
Scheduling the first ultrasound (ACOG recommends 7-13 weeks 6 days for dating).
Planning maternity leave, work handover, and family support logistics.
Timing prenatal screening tests (NIPT 10w, quad screen 15-22w, anatomy scan 18-22w).
Determining when to start counting kicks (typically 26-28 weeks for high-risk, 28 for low-risk).
Calculating gestational age for clinical decisions (steroids for fetal lung maturity 24-34w).
Setting the induction discussion window (ACOG offers induction at 39w for low-risk first births).

Limitations & Disclaimer

Naegele’s Rule assumes a regular menstrual cycle and accurately recalled LMP. Irregular cycles, recent hormonal contraception, breastfeeding, or unsure dates make the estimate unreliable; first-trimester ultrasound is preferred. Only ~5% of babies arrive on the EDD; the natural birth window spans 37-42 weeks. This tool is not medical advice; consult your obstetrician or midwife for prenatal care. See our disclaimer for full terms.

Frequently Asked Questions

How accurate is Naegele’s Rule?

About 5% of babies arrive on their exact due date; the 280-day estimate has a standard deviation of ~12-14 days. A first-trimester ultrasound (CRL at 7-13 weeks) narrows the margin to ±5-7 days and is the ACOG-preferred method when LMP is uncertain.

Why does cycle length matter?

Ovulation typically occurs 14 days before the next period. In a 28-day cycle, that’s day 14; in a 35-day cycle, day 21. Naegele’s standard 280-day rule assumes day 14 ovulation. For other cycles, the EDD shifts by the same number of days as the cycle differs from 28.

Should I rely on LMP or ultrasound for the EDD?

ACOG prefers the LMP-based EDD unless the first-trimester ultrasound differs by more than 5-7 days. At 7-13 weeks, ultrasound dating is most accurate (±5-7 days); accuracy drops to ±14 days in the 2nd trimester and ±21 days in the 3rd.

When is a baby considered full term?

ACOG defines full term as 39 weeks 0 days to 40 weeks 6 days. Births at 37w0d-38w6d are “early term” (higher risk of respiratory and feeding issues); 41w0d-41w6d are “late term”; ≥42w0d are “post term” and induction is typically offered at 41w.

What if my cycles are very irregular?

Naegele’s Rule is unreliable for cycles varying by >7 days between months. Use the earliest positive pregnancy test (HCG detectable ~10 days after conception), the date of intercourse if known, or schedule a first-trimester ultrasound for accurate dating. IVF pregnancies date from embryo transfer.

Does IVF use the same due date calculation?

No. IVF pregnancies date from embryo transfer: fresh 3-day transfer + 266 days, fresh 5-day blastocyst + 261 days, or frozen transfer + 266/261 days + embryo age. The first-trimester ultrasound still confirms. See ACOG Committee Opinion #700 for full guidance.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team