Due Date Calculator Guide: How Your Estimated Due Date Is Calculated

How Due Date Is Calculated

Your estimated due date (EDD) — also called estimated date of confinement (EDC) — is the projected date of delivery assuming a 280-day (40-week) pregnancy from the first day of your last menstrual period (LMP). This is a statistical average, not a prediction of when your specific baby will arrive.

The 280-day model assumes a regular 28-day cycle with ovulation on day 14. In reality, cycles vary from 21–35 days and ovulation timing varies within each cycle, which is why EDD is an estimate rather than a deadline.

Calculate yours now: our due date calculator accepts LMP date, cycle length, or known conception date to give you the most personalized estimate.

Naegele's Rule

The standard medical formula for calculating EDD is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812:

EDD = LMP + 7 days − 3 months + 1 year (or equivalently, LMP + 280 days)

Example: LMP = February 1, 2026 → Add 7 days = February 8 → Subtract 3 months = November 8 → Add 1 year = November 8, 2026. EDD = November 8, 2026.

Naegele's Rule assumes a 28-day cycle. For cycles longer than 28 days, add the extra days. For cycles shorter than 28 days, subtract the difference. A 32-day cycle shifts EDD 4 days later; a 24-day cycle shifts it 4 days earlier.

Trimester Breakdown

Pregnancy is divided into three trimesters. Weeks are counted from the first day of LMP, not conception:

A full-term pregnancy is 39–40 weeks. Early term = 37–38 weeks, late term = 41 weeks, post-term = 42+ weeks. Pre-term is before 37 weeks. These distinctions matter for neonatal health outcomes.

How Accurate Is the EDD?

Only 4–5% of births occur on the exact EDD. Most births occur within 2 weeks before or after. Research shows:

• ~70% of spontaneous labors begin within ±2 weeks of EDD
• ~90% begin within ±3 weeks of EDD
• First-time mothers (nulliparas) tend to deliver slightly later than the EDD on average
• Women who have given birth before tend to deliver closer to or before EDD

Ultrasound dating in the first trimester (crown-rump length measurement) is more accurate than LMP dating when cycles are irregular. First-trimester ultrasound has an accuracy of ±5–7 days. Third-trimester ultrasound is less accurate (±3 weeks).

LMP vs. Conception Date Dating

There are two reference points for calculating EDD:

• LMP (Last Menstrual Period): The standard clinical reference. Pregnancy "begins" 2 weeks before conception in LMP dating because the LMP date is easier to recall. Gestational age is counted from LMP.
• Conception date: Actual fertilization date, estimated as LMP + 14 days for 28-day cycles. If you know your conception date (from ovulation tracking, IVF transfer date, etc.), you can calculate EDD more precisely. Our conception date calculator works in both directions.

Gestational age (from LMP) = embryonic age (from conception) + 2 weeks. When doctors say "you are 8 weeks pregnant," they mean 8 weeks from LMP — the embryo is approximately 6 weeks old.

Track your cycle to improve conception date accuracy: our ovulation calculator predicts your fertile window and estimated ovulation day, and our period calculator helps you track cycle patterns over time.

Ovulation, Conception, and Due Date Timing

Understanding the sequence of events clarifies why the math works:

• Day 1: First day of LMP (menstruation begins)
• Day ~14: Ovulation (for 28-day cycle). Egg is viable for 12–24 hours post-ovulation.
• Day ~14–15: Fertilization occurs if sperm (viable 3–5 days) meets egg
• Day ~21: Implantation of blastocyst into uterine lining
• Week 4 from LMP: Missed period — earliest pregnancy detection point
• Week 40 from LMP: Estimated due date

If your cycle is not 28 days, adjust: a 35-day cycle means ovulation on day ~21, shifting your conception date and EDD 7 days later than a standard calculation. Our due date calculator automatically adjusts for your cycle length.

Common Due Date Mistakes

Due Date FAQs