Getting the Due Date Right: How Online Calculators Can Help

Knowing your due date when you’re expecting can give you a target to count down to. But pregnancy rarely goes precisely according to plan. Read on to learn how online due date calculators work, what impacts their accuracy, and why you should view any estimated due date as a general guideline rather than a precise prediction.

When you first see those two lines on the pregnancy test, one of the first things you’ll probably do (after freaking out, calling your partner, and scheduling a doctor’s appointment) is pull out your phone and start looking up your due date. BabyCenter, The Bump, What to Expect - they’ve all got pregnancy due date calculators that promise to tell you when you can expect your little one to arrive.

As an expectant mom, having an estimated due date can give you a sense of when the big day might come. It lets you start wrapping your head around the timing and logistics of going on maternity leave, taking time off work for delivery and recovery, and preparing your home for a newborn. Family and friends also want to know the due date so they can plan to take time off or arrange travel around the birth.

But it’s important to understand that online due date calculators and even estimates from your doctor are not a precise prediction or guarantee of exactly when your baby will be born. In fact, only around 5% of babies arrive on their actual estimated due date. So while due date estimates can be useful, they should be viewed as more of a general ballpark figure than a promise.

How Do Online Pregnancy Due Date Calculators Work?

Due date calculators online work by using one of several different methods to estimate when a baby will be born:

Last Menstrual Period: Most due date calculators will first ask you for the date that your last menstrual period started. This is often abbreviated as LMP. Based on the average length of a menstrual cycle, the calculator can estimate a due date that is 280 days (40 weeks) from the first day of your LMP.
Ultrasound Measurements: Some calculators allow you to enter information from early ultrasounds, which provide more precise measurements of the fetus to estimate the due date.
IVF Transfer Date: For women who became pregnant through IVF, some calculators can estimate the due date based on the embryo transfer date instead of LMP.
Conception Date: If you know the approximate date that you conceived, some due date calculators allow you to enter this date to derive an estimated due date.

Here are some of the most popular online pregnancy due date calculators:

American Pregnancy Association: This calculator estimates your due date based solely on the first day of your last period.
NHS Due Date Calculator: The UK’s National Health Service calculator allows you to enter your LMP date or IVF transfer date.
What to Expect: This due date calculator asks for your LMP date but also allows you to enter estimated conception date or dates from an early ultrasound.
BabyCenter: BabyCenter’s calculator uses LMP, date of conception, or ultrasound measurements to provide various due date estimates.
Calculator.net: This calculator can estimate your due date based on LMP, conception date, ultrasound, or IVF transfer date.

While these online calculators provide a useful ballpark estimate, it’s important to note they make predictions based on averages. Your own pregnancy journey may deviate from the norm for many reasons.

What Impacts the Accuracy of Due Date Calculators?

There are several factors that can affect the accuracy of a due date prediction from an online calculator or even your doctor:

Irregular Periods: Due date estimates based on a last menstrual period assume you have a typical cycle length. But if your cycles are irregular, the LMP date may not line up with actual timing of conception.
Conception Date Uncertainty: You may not know the exact date you conceived, which introduces some uncertainty into due date estimates.
Varying Menstrual Cycle Lengths: The average menstrual cycle is 28 days, but normal cycle length can range from 21 to 35 days. Variations in your own cycle impact accuracy.
Different Growth Rates: Not every fetus grows at the same rate in utero. Growth rate impacts the accuracy of estimates based on ultrasound measurements.
Mistaken LMP Date: It’s easy to be off on recalling the exact date your last period started, which skews LMP-based estimates.

According to the Cleveland Clinic, due to these and other factors, due date calculators can be off by up to 2 weeks in either direction[1]. That’s why healthcare providers will examine you and perform ultrasounds to confirm dates rather than relying solely on online calculators.

Ultrasounds Give Doctors More Precision

Once you get deeper into your pregnancy journey and start visiting your obstetrician, they will use methods beyond online due date calculators to estimate your due date. This often includes:

Measuring Uterus Size: As the pregnancy progresses, your doctor will measure the fundal height (the top of the uterus) to estimate gestational age.
First Trimester Ultrasound: Also called a dating scan, this first ultrasound (usually done between 8-13 weeks) provides the most accurate estimate of the due date[2]. The ultrasound measures the crown rump length of the fetus to derive the gestational age and due date.
Second Trimester Ultrasound: A more detailed anatomy scan ultrasound around 18-22 weeks provides additional information to confirm dates and fetal development.

According to What to Expect, first trimester ultrasounds have an accuracy of within 3-5 days when done before 14 weeks, and ultrasounds are more precise than LMP for due date predictions[3]. Doctors will take ultrasound due dates into account when making a final due date determination.

Don’t Expect Baby To Appear Precisely on the Due Date

Whether you used an online calculator or got a professional due date estimate from your doctor, remember that this date is merely a rough estimate and not a precise prediction. There are normal variations in the length of a pregnancy term.

Only about 4-5% of babies are actually born on their estimated due date. About half are born in the week before or after the due date, and some arrive more than 2 weeks early or late[4].

According to the Mayo Clinic, a pregnancy that results in a live birth can normally range from 38-42 weeks long[5]. Your healthcare providers will not consider your pregnancy to be “overdue” until you pass 42 weeks.

So while due dates can provide an exciting target day to count down to, try not to fixate too precisely on that calendar date. Focus instead on tracking your weekly progress and milestones during your pregnancy journey. And don’t be alarmed if your little one arrives earlier or later than your estimated due date.

Key Takeaways on Due Date Calculators

Here are some key points to remember about using online pregnancy due date calculators:

They provide estimates based on averages and should not be taken as a precise prediction.
Due to variations in menstrual cycles and fetal growth, the actual delivery date often deviates from the estimated due date.
Ultrasound measurements, especially in the first trimester, provide a more accurate estimate of gestational age than LMP calculations.
Don’t panic if you go into labor before or after your estimated due date, which is normal and expected.
A pregnancy lasting anywhere from 38-42 weeks is considered full term.

While due date estimates can be a useful general guide, the only way to know for sure when your baby will arrive is to wait for the big day! Try not to fixate too much on your estimated due date, since very few babies actually make their arrival on that precise date. Instead, enjoy watching your body change and your baby grow during each week of your amazing pregnancy journey.