Question 1

What is Pregnancy Due Date?

Accepted Answer

The estimated due date (EDD) is the predicted date of delivery, calculated as 280 days (40 weeks) from the first day of the last menstrual period (LMP) using Naegele's rule. The formula is: EDD = LMP + 7 days − 3 months + 1 year. For example, if LMP is March 1, the due date is approximately December 8. This assumes a regular 28-day menstrual cycle with ovulation on day 14. Only about 4–5% of babies are actually born on their due date — most arrive within a 2-week window before or after. Ultrasound dating in the first trimester (crown-rump length measurement) provides the most accurate due date estimation, often superseding the LMP calculation.

Question 2

What is the formula for Pregnancy Due Date?

Accepted Answer

The formula is: EDD = LMP + 7 days − 3 months + 1 year (Naegele's rule)

Question 3

How accurate is my due date?

Accepted Answer

Even with the best methods, only 4–5% of babies are born on the exact due date. It's best understood as an estimate within a 4-week window — most babies arrive between 38 and 42 weeks. First-trimester ultrasound provides the most accurate dating (±5–7 days). LMP-based calculation is accurate to about ±2 weeks, assuming regular cycles. If ultrasound and LMP dates differ by more than 5–7 days, the ultrasound date is typically used.

Question 4

Can my due date change?

Accepted Answer

Yes. If an early ultrasound suggests a different date than your LMP calculation — especially if the difference is more than 5–7 days — your healthcare provider will typically adjust the due date to match the ultrasound. This commonly happens when women have irregular cycles, don't know their exact LMP date, or conceived at a time other than mid-cycle. The due date is usually finalized by the 20-week anatomy scan.

Question 5

What if my cycle is not 28 days?

Accepted Answer

Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), ovulation likely occurred later — add the extra days to your EDD. A 35-day cycle means ovulation around day 21 instead of day 14, so add 7 days to the standard EDD. Conversely, shorter cycles (e.g., 24 days) mean earlier ovulation and subtracting days from the EDD. First-trimester ultrasound eliminates this uncertainty.

If LMP Is	Estimated Due Date
January 1	October 8
March 15	December 22
June 1	March 8 (next year)
September 20	June 27 (next year)

Method	Accuracy	Notes
First trimester ultrasound	±5–7 days	Most accurate — based on crown-rump length (CRL)
LMP calculation	±14 days	Assumes regular 28-day cycle
Second trimester ultrasound	±10–14 days	Less accurate as growth varies more
Third trimester ultrasound	±21 days	Used only if earlier dating unavailable

Trimester	Weeks	Key Developments
First	Weeks 1–12	All major organs form, heartbeat detectable (week 6), highest miscarriage risk
Second	Weeks 13–26	Gender determination, baby moves ("quickening"), anatomy scan (week 20)
Third	Weeks 27–40	Rapid growth, lungs mature, baby moves to head-down position

Pregnancy Due Date

Definition

Why is Pregnancy Due Date Important?

What is an Estimated Due Date?

How to Calculate Due Date

Naegele's Rule (Standard Method)

Dating Methods Accuracy

Pregnancy Timeline

Important Statistics

Formula

🔗 Related Calculators

Related Terms

Pregnancy Due Date — Frequently Asked Questions