Calculating Due Date

“When are you due?” It’s usually the very first question people ask when they find out about your pregnancy. For that matter, it’s often the very first question expecting moms want answered! We know that pregnancy lasts about nine months, but how exactly are due dates calculated?

Pregnancy wheel

The pregnancy wheel method of calculating the estimated due date (EDD) has been around for a long time, and it’s still the simplest and most widely used method. The basics are this: take the date of the first day of your last menstrual period and add 280 days (40 weeks). The wheel itself is a shortcut that, when you point the inner wheel to the date of your last period, the outer wheel points to your EDD.

Naegele’s rule

This is an easy math equation that follows the same 280-day guideline as the pregnancy wheel.

  • Take the first day of your last period.
  • Add seven days.
  • Subtract three months.

So, for example, if you last period started on July 1, you would add seven days (July 8) and then subtract three months—making your EDD April 8.


If you aren’t sure when you last had your period, or if your cycles are irregular, a measurement of your baby’s crown-rump length (CRL) may be taken by ultrasound. The CRL is the length of your baby from the top of their head to their bottom (the legs are usually curled up underneath). Early in the first trimester, the CRL can offer a very accurate estimate of the age of the baby, which your doctor can then use to calculate your EDD.