How accurate is this due date estimate?

It's a standard estimate used widely in early pregnancy, but actual delivery dates commonly vary by a couple weeks in either direction — an ultrasound-based estimate from a care provider is generally more precise as pregnancy progresses.

What if my cycle isn't a typical 28 days?

This calculator assumes a standard cycle length. If your cycle is significantly longer or shorter, your care provider may adjust the estimated due date based on ultrasound measurements instead.

What are the three trimesters?

The first trimester is weeks 1-12, the second is weeks 13-26, and the third is week 27 until birth (around week 40).

Pregnancy Due Date Calculator

Estimate your due date and current week of pregnancy based on the first day of your last menstrual period.

First day of last menstrual period

Enter the first day of your last period.

How It Works

Naegele's rule

The estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — the standard method used by most pregnancy calculators and many care providers.

Current week and trimester

Your current week is based on the number of days since your last period, and trimesters are grouped as weeks 1-12 (first), 13-26 (second), and 27+ (third).

Example Calculation

If your last period started on January 1, your estimated due date is around October 8 — about 40 weeks later.

Common Mistakes

Confusing 'weeks pregnant' with 'weeks since conception' — pregnancy is dated from the last period, about 2 weeks before conception typically occurs.
Treating the due date as exact — only a small percentage of babies are born on their exact estimated due date; a range of a couple weeks either side is normal.
Using an irregular cycle without adjustment — Naegele's rule assumes a fairly typical 28-day cycle.

Frequently Asked Questions

How accurate is this due date estimate?: It's a standard estimate used widely in early pregnancy, but actual delivery dates commonly vary by a couple weeks in either direction — an ultrasound-based estimate from a care provider is generally more precise as pregnancy progresses.
What if my cycle isn't a typical 28 days?: This calculator assumes a standard cycle length. If your cycle is significantly longer or shorter, your care provider may adjust the estimated due date based on ultrasound measurements instead.
What are the three trimesters?: The first trimester is weeks 1-12, the second is weeks 13-26, and the third is week 27 until birth (around week 40).

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