Why You Might Want to Use a Pregnancy Due Date Calculator (and How to Use It)

Content medically reviewed by Erin Roggenkamp, RNC-OB, CLC

You're in the exciting and nerve-wracking stage between a positive pregnancy test and your first prenatal appointment. Wondering what's next? Even before you see a pregnancy provider, a pregnancy due date calculator can help you learn some of the basics.

"A due date calculator is exactly what it sounds like—you provide the day of your last period and the calculator provides you with an estimated pregnancy due date," said Erin Roggenkamp, obstetrics nurse at Essentia Health. "A pregnancy calculator might also ask you for your average menstrual cycle length to provide a more accurate estimation."

Some calculators can also estimate your due date based on your conception date, if you know the specific date, or some even determine an estimated conception date based on your due date.

Using one of these calculators to determine an estimated due date can help you plan ahead, both for milestones along the way and for the big arrival.

The Benefits of Using a Pregnancy Due Date Calculator

An estimated due date can help you start planning for your pregnancy before your first prenatal appointment. You can go ahead and start taking prenatal vitamins if you haven’t already and maintaining a healthy lifestyle. You’ll also want to avoid alcohol, cigarettes and any other harmful substances.

“Whether you’re expecting a baby for the first time or the fifth, it can be helpful to have a target date in mind for a number of reasons,” Roggenkamp said. “For one, it can help you begin to plan for the ‘big picture’ of pregnancy, including logistics such as when you’ll need to take maternity leave and how long you have to put together the nursery.”

Bigger than that, though, it can also help ease your mind. Some pregnancy calculators are paired with a newsletter or an app, allowing you to sign up for weekly updates that keep you in the know about your stage of pregnancy. Essentia Health’s Care Companion, part of MyChart, personalizes your pregnancy experience by offering week-by-week updates on your baby’s size, as well as content, reminders, and support. 

This information can be handy to have even before your first prenatal appointment—and it’s also an exciting way of keeping up with the stages of pregnancy.

How Pregnancy Calculators Estimate Your Due Date

Most pregnancy calculators ask for the date of your last period. That’s because that’s the date when your pregnancy technically begins. Your pregnancy is calculated from the first day of your last menstrual period. As a result, the first two weeks of your “pregnancy” are actually time when you aren’t pregnant.

And, although most people think of pregnancy as lasting nine months, that’s not actually correct. When it feels like your pregnancy is going on and on, you can correctly tell people that you’ll have been pregnant for 10 months by the time the baby arrives.

The average length of time you’ll be pregnant is 280 days, but it’s calculated as 40 weeks for the purpose of your due date, since it counts back to your last period. Most individuals will deliver between the 38th and 41st weeks of pregnancy.

An Estimated Due Date From Your Pregnancy Provider

When you meet with your pregnancy provider of choice for your first prenatal appointment, your provider will determine your baby’s gestational age and provide an estimated due date, usually based on the date of your last period and an ultrasound scan.

The estimated due date, or EDD, you’re given at this appointment may align with what was provided by a pregnancy calculator, but it may also be slightly different based on what’s seen on an ultrasound. The ultrasound scan performed during one of your first prenatal appointments can also be used to determine gestational age and due date, especially if you aren’t exactly sure of the date of your last period.

When your provider performs this ultrasound, they’re looking specifically at what’s known as the crown-rump length of your baby. That’s a measurement from your baby’s head down to the bottom of its buttocks, which is considered the most accurate measurement of gestational age during the first eight to 12 weeks of pregnancy.

Planning Ahead

Why is the due date so important? It guides every aspect of your prenatal care.

“With your estimated due date in mind, we can recommend a schedule of prenatal appointments, important screenings, and other tests for both you and your baby,” Roggenkamp said. “Knowing your stage of pregnancy also gives your provider important information about your health.”

That includes which symptoms are normal and which ones are not. That’s because some symptoms, such as spotting, are less worrisome during certain times of pregnancy than during others.Once your estimated due date is determined, it usually doesn’t change. The American College of Obstetricians and Gynecologists recommends that revisions to an EDD be rare and based on ultrasound imaging later in pregnancy.