Calculate My Due Date: Pregnancy Due Date Methods

Searching calculate my due date means you want an estimated pregnancy due date. An estimated due date (EDD) is not a precise delivery date — it is the midpoint of a normal delivery window, around which most pregnancies deliver. Understanding how it is calculated helps you use pregnancy tracking tools accurately and have informed conversations with your healthcare provider.

Use our Pregnancy Calculator for a quick estimate.

The Most Common Method: Last Menstrual Period (LMP)

The standard clinical method for estimating due date is based on the first day of the last menstrual period.

Estimated due date = First day of last menstrual period + 280 days

280 days = 40 weeks

This is based on a 28-day cycle with ovulation on day 14. Pregnancy weeks are counted from the LMP, not from conception, which is why most pregnancies are described as 40 weeks even though fertilization typically occurs around week 2.

Example:

Last menstrual period started: January 1, 2026

Due date: January 1 + 280 days = October 8, 2026

Naegele’s Rule

The classic formula taught in medical training is Naegele’s Rule:

EDD = LMP + 9 months + 7 days

(or equivalently: subtract 3 months and add 7 days)

Example:

LMP: January 1, 2026

January + 9 months = October 1

October 1 + 7 days = October 8, 2026

Same result as adding 280 days. The two are equivalent for a standard calendar year.

From Conception Date

If you know approximately when conception occurred:

EDD = Conception date + 266 days (38 weeks)

Why 266 days rather than 280? Because LMP dating counts 14 days before ovulation/conception. If you know the actual conception date, you add 266 days instead.

Example:

Conceived on: January 15, 2026

EDD = January 15 + 266 days = October 8, 2026 (approximately)

Conception date estimates are less precise than LMP dates because ovulation varies and conception cannot be observed directly in most cases.

From IVF Transfer Date

For pregnancies from IVF (in vitro fertilization), the due date can be calculated from the embryo transfer date:

• Day 3 embryo transfer: EDD = transfer date + 263 days
• Day 5 blastocyst transfer: EDD = transfer date + 261 days

These calculations are more precise for IVF pregnancies because the exact fertilization date is known.

Example (Day 5 transfer):

Transfer date: January 20, 2026

EDD = January 20 + 261 days = October 8, 2026 (approximately)

Ultrasound Dating

Ultrasound dating is often more accurate than LMP dating, especially for women with:

• Irregular cycles
• Uncertain LMP date
• Cycles significantly longer or shorter than 28 days
• Recent hormonal contraception use

Early pregnancy ultrasounds (8–13 weeks) measure the crown-rump length (CRL) of the embryo. This measurement is highly predictive of gestational age and allows the due date to be adjusted if it differs from the LMP estimate.

If ultrasound dating differs from LMP dating by:

• Less than 7 days: the LMP date is usually kept
• 7 days or more: the ultrasound date is often used instead

After about 20 weeks, ultrasound dating becomes less accurate for adjusting the due date.

Due Date Based on Cycle Length

Standard LMP calculations assume a 28-day cycle. If your cycle is consistently different, your actual ovulation and conception timing differ from the standard.

Adjustment for non-28-day cycles:

If your cycle is longer than 28 days, ovulation occurs later, and your due date is likely later.

If your cycle is shorter than 28 days, ovulation occurs earlier, and your due date may be earlier.

Rough adjustment:

EDD adjustment = Cycle length − 28 days

Example: 35-day cycle — ovulation is about 7 days later than standard:

Standard EDD + 7 days = adjusted EDD

This is one reason ultrasound dating can be more accurate for women with irregular cycles.

What Week Am I In?

Pregnancy weeks are counted from the LMP date:

Current pregnancy week = (Days since LMP) ÷ 7

Example:

LMP: January 1, 2026

Today: March 10, 2026

Days since LMP: January (31−1) + February (28) + March 1–10 = 30 + 28 + 10 = 68 days

Pregnancy week: 68 ÷ 7 = 9.7 → 9 weeks and 5 days

Key Pregnancy Milestones

Week	Milestone
4	Missed period, positive pregnancy test
6–8	Heartbeat typically detectable on ultrasound
10–12	First trimester ends
12–13	First trimester screening window
18–22	Anatomy ultrasound
28	Second trimester ends / third begins
37	Considered full term (early term: 37–39 weeks)
39–40	Full term delivery window
40	Estimated due date
41+	Post-term; monitoring increases

Is the Due Date Exact?

No. A due date is an estimate. Only about 4–5% of babies are born on their exact due date. The majority of births happen between 37 and 42 weeks of pregnancy.

The due date represents the middle of the expected delivery window — it is a target, not a deadline.

Common Mistakes When Calculating Due Date

Using the wrong date as LMP — the first day of the last period is the start of bleeding, not the end.

Confusing LMP date with conception date — these are usually about 2 weeks apart.

Assuming 9 months exactly — a 40-week pregnancy is about 9 months and 1 week when measured in calendar months.

Not updating after ultrasound — if your first ultrasound suggests a different gestational age, discuss whether to update the EDD with your provider.

The Bottom Line

The most common method to calculate a due date is: add 280 days (40 weeks) to the first day of the last menstrual period. For irregular cycles or IVF pregnancies, alternative methods or ultrasound dating may give a more accurate result.

Use the Pregnancy Calculator for a quick estimate based on LMP or conception date.

Medical note: This article is informational only. Due date calculations are estimates and not a substitute for prenatal care. Always confirm pregnancy dating and health information with a qualified healthcare provider.

How to Calculate: Step-by-Step Guide

1

Start with LMP

Use the first day of your last menstrual period when known.

2

Add 280 days

A common estimate adds 40 weeks, or 280 days, to the LMP date.

3

Adjust with clinical guidance

Ultrasound dating, cycle length, or IVF details may change the estimate.