How to Calculate Baby Delivery Due Date

Calculating your baby's due date is an important part of pregnancy planning. There are several reliable methods to determine when your baby is expected to be born, including the Last Menstrual Period (LMP) method and ultrasound dates. This guide explains each method in detail and provides a calculator to help you estimate your due date.

Last Menstrual Period (LMP) Method

The LMP method is the most common way to estimate a due date. It works by adding 280 days (40 weeks) to the first day of your last menstrual period. This method assumes a standard 28-day menstrual cycle and a 266-day gestation period.

Formula: Due Date = LMP Date + 280 days

For example, if your last menstrual period started on January 1, 2024, your estimated due date would be October 31, 2024.

Note: The LMP method is less accurate than ultrasound dates, especially for pregnancies with irregular cycles or multiple babies.

Ultrasound Dates

Ultrasound dates are more accurate than the LMP method because they measure the baby's development. The most common ultrasound dates are:

Crown Rump Length (CRL): Measured at about 6-13 weeks, this ultrasound estimates the baby's age based on the crown rump length.
First Trimester (NT) Ultrasound: Measured at about 11-14 weeks, this ultrasound measures the nuchal translucency to estimate the baby's age.
Second Trimester Ultrasound: Measured at about 18-22 weeks, this ultrasound estimates the baby's age based on the biparietal diameter and femur length.

Formula: Due Date = Ultrasound Date + (40 weeks - Ultrasound Weeks)

For example, if you had a first trimester ultrasound at 12 weeks and 2 days, and the ultrasound estimated the baby's age as 12 weeks and 1 day, your due date would be calculated as follows:

Example Calculation: Ultrasound Date = 12 weeks and 1 day
Due Date = Ultrasound Date + (40 weeks - 12 weeks) = 12 weeks and 1 day + 28 weeks and 3 days = 40 weeks and 4 days

Comparison Table

Here's a comparison of the LMP method and ultrasound dates:

Method	Accuracy	When to Use
LMP Method	Less accurate (about ±10 days)	When you don't have access to ultrasound
Ultrasound Dates	More accurate (about ±3-5 days)	When you can get an ultrasound

Frequently Asked Questions

How accurate is the LMP method?

The LMP method is less accurate than ultrasound dates, with an estimated accuracy of about ±10 days. It's best used when you don't have access to ultrasound.

When should I get an ultrasound for the most accurate due date?

You should get an ultrasound as early as possible in the first trimester, ideally between 11-14 weeks, for the most accurate due date.

What if my menstrual cycle is irregular?

If your menstrual cycle is irregular, the LMP method may not be accurate. In this case, it's best to get an ultrasound as soon as possible.

Can the due date change after an ultrasound?

Yes, the due date can change after an ultrasound if the baby's age is estimated differently than expected. It's important to follow up with your healthcare provider.