How to Calculate Your Pregnancy Due Date

TL;DR

• The standard method: Take the first day of your last menstrual period, add 280 days (40 weeks). That's your estimated due date.
• Only about 5% of babies arrive on their due date. It's an estimate, not a deadline.
• Ultrasound dating is more accurate, especially in the first trimester when it can pin down gestational age within a few days.
• "Full term" is 39-40 weeks. Babies born at 37-38 weeks are "early term," and 41 weeks is "late term."

The Naegele's Rule Method

The most common way to estimate your due date is Naegele's Rule:

1. Take the first day of your last menstrual period (LMP)
2. Add 7 days
3. Subtract 3 months
4. Add 1 year

For example, if your last period started on March 1, 2026:

• Add 7 days → March 8
• Subtract 3 months → December 8
• Your estimated due date is December 8, 2026

This method assumes a 28-day menstrual cycle and ovulation on day 14. If your cycles are longer or shorter, the estimate may be off.

Other Ways to Calculate

From Conception Date

If you know the exact date of conception (from IVF or tracking ovulation), add 266 days (38 weeks). This method is more precise because it removes the guesswork about when ovulation occurred.

From Ultrasound

First trimester ultrasounds (before 13 weeks) can estimate your due date based on your baby's size, specifically the crown-rump length. This is considered the most accurate dating method and can be precise to within 5-7 days.

Your provider may adjust your due date based on ultrasound measurements, especially if it differs from the LMP calculation by more than a week.

From IVF Transfer Date

If you conceived through IVF:

• Day 3 embryo transfer: Add 263 days to the transfer date
• Day 5 embryo transfer (blastocyst): Add 261 days to the transfer date

Your fertility clinic will give you your official due date.

What Your Due Date Actually Means

Your due date is the midpoint of a window, not a target date. Here's how doctors categorize timing:

Key fact: Only about 5% of babies are born on their actual due date. Most arrive within a window of 2 weeks before to 2 weeks after. First babies tend to arrive a bit later on average.

Why Due Dates Get Changed

Your provider may change your due date if:

• An early ultrasound shows a gestational age that differs from LMP calculations by more than a week
• You have irregular periods, making LMP-based dating unreliable
• You're unsure of your last period date

Once a due date is established by early ultrasound, it generally isn't changed again — even if later ultrasounds show different measurements (because babies grow at different rates in the second and third trimester).

Counting Weeks and Trimesters

Pregnancy is counted from the first day of your last period, not from conception. This means:

• Week 1-2: You're not actually pregnant yet (this is before conception)
• Week 3: Conception typically occurs
• Week 4: Implantation, around the time you'd miss your period
• Week 40: Your due date

The trimesters break down as:

• First trimester: Weeks 1-13
• Second trimester: Weeks 14-27
• Third trimester: Weeks 28-40

When to Ask Your Provider

• If you're unsure of your last period date
• If your cycles are very irregular
• If you want to understand how your due date was calculated
• If you're approaching or passing your due date and want to discuss your options

Sources

• ACOG — Methods for Estimating Due Date

Related Articles

• I Just Found Out I'm Pregnant — Now What?
• First Trimester: The Complete Guide
• Your First Prenatal Appointment