Pretty much every mama out there has found themselves searching the internet for a due date calculator. Whether you are trying to conceive with specific timing in mind, or are already pregnant and curious to know when your little bundle will arrive, thinking about due dates is exciting—and gives you a thrilling countdown.

When trying to calculate your due date, your medical team will often start with your last menstrual period, or LMP (the first day of your last period, to be exact). From there, we can estimate the due date based on a simple mathematical equation (more on that below).

But what if you don't remember the first day of your last period? Or what if you are someone who doesn't get regular periods? No problem.

As I share in The Motherly Guide to Becoming Mama, it's common for women not to know when their LMP was. If that's the case, your doctor or midwife will likely recommend what's known as a dating ultrasound to help determine how far along you are.

Let's discuss what due dates are, and most importantly, how to determine when your due date is. Try our due date calculator below!

What is a due date?

Due dates can encompass a range of terminologies, all referring to the same thing. You might hear:

  • Estimated date of delivery (EDD)
  • Estimated date of childbirth (EDC)
  • Estimated date of confinement (EDC) (Wait, what? Back in the day, women were confined to a bed or hospital when they gave birth, hence the word confinement.)
  • Guess day

Whatever phrasing you use, your due date is the day that we estimate your baby will be born.

Related: The new mama's guide to pregnancy symptoms

How long is pregnancy?

Pregnancies last approximately 280 days, which is 40 weeks, or about 10 months—not nine like we traditionally hear. Your pregnancy is considered full-term after 37 weeks. Delivery prior to 37 weeks is considered preterm.

  • At 37 weeks, the pregnancy is considered early-term (technically full-term, but on the early side of it)
  • At 41 weeks, the pregnancy is considered late-term
  • At 42 weeks, the pregnancy is considered post-term

When is my due date?

There are a few ways to calculate your due date. The first is based on the first day of your last menstrual period (LMP). Here is the formula:

  • Write down the first day of your last LMP
  • Subtract three months
  • Add seven days

So, say your LMP was on April 10th:

  • Subtracting three months from April brings you to January.
  • Adding seven days to 10 gives you 17.
  • Your due date will be January 17th.

You can also use a due date calculator, which can also help if you know the day of intercourse, insemination or embryo transfer, or try our handy graphic below.

Related: September is by far the most popular birthday month—and here’s why

Using ultrasounds to determine your due date

For a variety of reasons, a lot of women don't know when their last period was (so don't worry if you're not sure, mama). A history of irregular periods, medications and plain ol' forgetting are all normal reasons to be unsure.

transvaginal ultrasound (where a lubricated ultrasound wand is inserted into the vagina) can help to pinpoint your estimated due date, especially when done in the first trimester. The ultrasound technician will measure the size of the embryo and use the information gathered to determine your due date.

Surprisingly, as your pregnancy progresses, it becomes harder to calculate the gestational age and size of the fetus with an ultrasound (the bigger they get, the more inaccurate the measurements are).

Related: 9 essential questions to ask during your first prenatal visit

How accurate are due dates?

In short, due dates are not very accurate. In fact, only about 4% of women will give birth on their due date. Note that due dates are not expiration dates! Most first-time moms will give birth 3-5 days past their due date, and a great many go longer than that still.

Going past your due date can be frustrating. Your provider will discuss all of your options, like waiting for labor to start on its own versus scheduling an induction with you. (Psst: If that's you, here's how to give yourself grace when you go past your due date.)

Ultimately, due dates are really exciting to think about—but my recommendation is to try to think of them as an approximation. Instead of saying, "I'm due on May 15th," maybe try, "I'm due sometime in May!"

Mama, your baby will come, and when they do, it will be the most perfect date of all: Your baby's birthday!

Try our due date calculator

For all those planner mamas-to-be: Our due date calculator can help you figure out the day your baby is due to be born based on your conception month.

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A version of this post was originally published on June 26, 2018. It has been updated.