How is a due date calculated?

The most common method is Naegele's Rule, which calculates the due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This calculator also allows calculation based on the date of conception, which adds 266 days (38 weeks).

Is my due date a guarantee?

Absolutely not. The due date is an estimate. Only about 5% of babies are born on their exact due date. A normal, full-term pregnancy can last anywhere from 38 to 42 weeks.

What if my cycles are irregular or longer/shorter than 28 days?

If you use the LMP method, entering your average cycle length allows our calculator to adjust the due date for better accuracy. If your cycles are very irregular, an early ultrasound performed by your doctor will provide the most accurate due date.

Pregnancy Due Date Calculator

MEDICAL DISCLAIMER: This calculator provides an estimate for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.

Your Estimated Due Date is:

Your Pregnancy Timeline

Current Gestational Age:
Start of 2nd Trimester:
Start of 3rd Trimester:
Conception Likely Occurred Around:

How is the Due Date Calculated?

This calculator uses standard methods to estimate your due date:

Last Menstrual Period (LMP): This is the most common method. It uses Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last period. This assumes a 28-day menstrual cycle. Our calculator adjusts this for your specific cycle length for better accuracy.
Conception Date: If you know the exact date of conception (from ovulation tracking or IVF), this can be a very accurate method. The calculation adds 266 days (38 weeks) to your conception date to find the estimated due date.

Remember, the due date is just an estimate. A full-term pregnancy can range from 38 to 42 weeks.

The Surprising History of Naegele's Rule

The go-to method for calculating a due date, Naegele's Rule, has a fascinating history. It's named after German obstetrician Franz Karl Naegele, who published it around 1812. However, he didn't invent it from scratch!

The rule was actually based on the work of another scholar, Hermann Boerhaave, who decades earlier figured out that a normal human pregnancy lasts about 10 lunar months. Naegele simplified this into a practical rule for doctors: take the first day of the last menstrual period, subtract three months, and add seven days. This simple calculation has remained the standard for over 200 years, proving to be an incredibly durable and useful tool in obstetrics, even in the age of modern technology.

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Frequently Asked Questions (FAQ)

How accurate is this due date calculator?

The calculator provides a reliable estimate based on the information you provide. However, it is only an estimate. Your doctor may adjust your due date based on the results of an early ultrasound, which is considered the most accurate method for dating a pregnancy.

What if I have irregular periods?

If your cycles are irregular, the LMP method can be less accurate. If you know your conception date, using that method will give a better estimate. Otherwise, an ultrasound will be the best way for your healthcare provider to determine your due date.

Can I plan my baby's birth for the exact due date?

It's highly unlikely. Only about 5% of babies are born on their estimated due date. It's more helpful to think of your due date as the center of a "due window" of several weeks during which your baby is likely to arrive. A normal, full-term delivery can happen anytime between 38 and 42 weeks of gestation.