Due Date Calculator: When Will My Baby Arrive?

Consider yourself warned: “When is your due date?” is a question that everyone you know—and many strangers, too—will ask you in the coming months.

You'll learn an estimated due date (EDD) during your prenatal appointments. If you're too eager to wait that long (we don't blame you!), there's an easy way to figure it out yourself beforehand. It simply involves using an online pregnancy due date calculator or a conception calculator.

Keep reading to learn about the importance of a due date, and how to use a due date calculator to track the exciting months ahead of you.

Why Is Your Due Date Important?

Knowing your estimated due date isn’t just important for conversational purposes. It's a crucial part of someone’s prenatal care. “It determines how far along they are at any given point in the pregnancy, so it affects when they need certain blood tests, ultrasounds, and other examinations," says Noopur Ghade, MD, an OB-GYN at Tufts Medical Center.

And though the due date isn't a definite predictor of your baby's arrival, it gives parents an estimate of when they might deliver. “It can be helpful for preparing the home for the baby," says Dr. Ghade. Parents can also use their baby's due date to plan for child care, work, travel, and other logistical concerns.

Plus, your due date will provide the information you need for a proper countdown, so you can anticipate the day you'll finally meet your little one!

How Does the Due Date Calculator Work?

A due date calculator works by determining when your baby will arrive. It does this in a couple of ways: either by using your last menstrual period (LMP) or your conception date.

Using Last Menstrual Period (LMP)

Knowing the start date of your last menstrual period (LMP) before conception can help when calculating your due date. Indeed, studies show that an accurate LMP is the best estimator to determine the due date.

But there's one small catch: This is based on someone who has regular menstrual cycles, says Salena Zanotti, MD, an OB-GYN at Cleveland Clinic. Your periods are considered "regular" if they come every 24 to 38 days, with ovulation generally happening 14 days before your next period arrives. If your periods are spaced longer or shorter than this—or if they come at an unpredictable time each month—they're considered irregular.

Irregular periods are common, but they can impact the accuracy of a due date calculator when using your last menstrual period.

Using Your Conception Date

A due date calculator can also work by using the date of conception—something you may know if you’ve been tracking your fertile window while trying to conceive. You might've done this with an ovulation calculator, monitoring your cervical mucus, taking your basal body temperature, or using an ovulation predictor kit (OPK).

Not everyone will know their exact conception date, though, especially if they aren't tracking ovulation. It's also important to note that the conception date won't necessarily be the same day you had sex. Sperm can live up to five days in the reproductive tract—and an egg lives 12 to 24 hours—meaning you can conceive by having sex in the days leading up to ovulation.

Are Due Date Calculators Accurate? 

Pregnancy due date calculators can't guarantee exact timelines, but they can provide a general guideline. Research shows that only about 4% of people give birth on their estimated due date, and about 70% of people will deliver within 10 days of their EDD.

Conception calculators are often most accurate for people with typical 28-day cycles. If a person has a regular menstrual cycle, and they know the first date of their last period, they can somewhat rely on due date calculators, says Samuel Campbell, MD, an OB-GYN with Bon Secours at Richmond Community Hospital.

On the other hand, if a person has irregular menstrual cycles, this means ovulation may not occur on a predictable date. Therefore the due date calculator may not be as accurate. "It’s best to confirm your due date with your OB-GYN via an ultrasound" in this case, says Dr. Campbell.

Factors That Might Affect Your Due Date 

Several factors can affect the accuracy of the due date calculation, including the regularity of your periods, your ovulation date, and your pregnancy length.

A pregnancy sometimes ends earlier than expected, and the following factors might increase your risk:

• Maternal age of 35 or older
• The parent or unborn baby has a higher weight
• You're carrying multiples
• Medical complications like preeclampsia, premature rupture of membranes (PROM), gestational diabetes, placental problems, concerns about amniotic fluid levels, infections, and more

Other Ways to Determine Your Due Date 

A due date calculator is just one piece of the prenatal puzzle. It should be used in conjunction with other tools to make the closest estimate of when your baby will arrive. Here are additional ways to predict your baby's due date.

Ultrasound Scan

Some pregnant people get a first-trimester pregnancy ultrasound, which calculates the estimated gestational age and the estimated due date (EDD) of the fetus. "The EDD determined by the ultrasound measurements is then compared to EDD determined by the last menstrual period," says Dr. Campbell.

After comparing these two numbers, your health care provider may choose to update your baby's due date, if they think it's necessary. What's the reason for a possible discrepancy? "You may have ovulated a little earlier or later than expected," says Dr. Ghade.

Note that, after this early pregnancy ultrasound, the due date is likely not going to change again. "The earliest ultrasound is the most accurate" for due date calculation, says Dr. Ghade.

Fertility Treatments Like IVF

Due date calculations work differently if you’ve undergone fertility treatments like in vitro fertilization (IVF). In this case, "the reproductive endocrinologist determines the due date based on when the IVF process occurred," says Dr. Campbell. In other words, they'll use the date of embryo transfer (when the fertilized embryo was implanted into the uterus) to determine the due date.

Frequently Asked Questions

How soon can you determine the due date?

As soon as you know you're pregnant, you can determine the due date with an online calculator— "but it's preferable to confirm this with an early ultrasound as well,” says Dr. Ghade.

How long is a typical pregnancy?

A typical pregnancy lasts 280 days (40 weeks) from the first day of your last menstrual period. If you're going off the ovulation date, it lasts about 266 days (38 weeks) after conception.

Can your due date change?

It's rare for your due date to change, but it might happen after your first-trimester ultrasound. “Sometimes, your early ultrasound will determine a due date that's different from the one you or your doctor may have calculated from your last menstrual period," says Dr. Ghade.

Can you plan your due date?

It's impossible to schedule an exact due date for your baby—though you can time your conception attempts about 266 days ahead of your preferred due date and hope for the best!