Due Date Calculator

Pregnancy due date calculator video

Learn how to calculator your pregnancy due date in this video.

When is my baby’s due date?

So you got your positive pregnancy test, you’re feeling some early signs of pregnancy, and now you’re wondering, “when is my baby’s due date?” We’ve got you covered with the Mama Natural due date calculator!

Enter your information in the due date calculator above and discover the best estimate for when your little bundle of joy will make his or her appearance.

How does this due date calculator work?

Because you may not know exactly when you ovulated or conceived, a due date calculator will typically calculate your estimated due date based on your last menstrual period (LMP).

Our online due date calculator uses a simple method to calculate your due date.

• Your due date is estimated to be 40 weeks after the first day of your LMP
• Your cycle is assumed to be 28 days long, with ovulation occurring at day 14
• Therefore the calculator adds 280 days (40 weeks) to your LMP

This method of due date calculation is known as Naegele’s rule (more info on this below).

My cycle isn’t 28 days. Will this due date calculator work for me?

Yes. The logic behind our pregnancy calculator works as follows:

• The average cycle length is 28 days
• If your cycle length is shorter, your due date will be earlier
  • For every day your cycle is shorter, your due date moves one day earlier
• Similarly, if your cycle is longer, your due date will be later
  • For every day your cycle is longer, your due date moves one day later

How do you calculate due date from conception?

If you know when you conceived, our pregnancy calculator calculates your due date by adding 38 weeks to the date of conception. This method of calculation may be more accurate than a LMP due date calculation if you have irregular or consistently longer or shorter cycles than 28 days.

What exactly is the date of conception?

The date of conception is the day that the egg and sperm meet.

Women who track their ovulation may know their exact date of conception. But for many women, date of conception can be tricky to pinpoint.

Sperm can live in a woman’s body for up to five days, and the ovum (egg) can live for up to 24 hours after being released. In other words, you have a six-day window where you could potentially get pregnant each month.

Do you already know your due date but want to know when you likely conceived? Try our reverse due date calculator.

What is an estimated due date (EDD)?

An estimated due date (EDD) is a “best guess” as to when baby might be born based on a due date calculator like this one.

However, only 4% of babies are born on their due date! Whereas 80% of babies are born within the window of two weeks before and two weeks after your due date calculator results. (See “due month” section below.)

What is “gestational age?” Can it be different than what the calculator shows?

Gestational age (GA) is the term used to describe how far along the pregnancy is and how long baby has been gestating (growing in the uterus).

If you get an ultrasound you may notice a “GA” on the image with a number of weeks and days. This figure is based on how the baby is measuring, not on your LMP, which the due date calculator uses.

It’s normal for these dates to not match up perfectly. If there are significant differences in the dates, your doctor may want to dig deeper to determine conception date. As a result, your midwife or doctor may change your due date based on the ultrasound gestational age.

Early ultrasounds are very accurate when dating a pregnancy and can be helpful if you don’t know your LMP or your periods are irregular.

Note that you don’t have to have an early ultrasound, especially if you are fairly certain of your cycle length and conception window. This study shows that early dating ultrasounds don’t change the incidence of induction.

How are the weeks of pregnancy calculated?

The 40 weeks of pregnancy begin on the first day of your last menstrual period.

This can be a little confusing because, for most people, conception doesn’t occur until day 14 of the menstrual cycle. So yes, you aren’t actually pregnant during those first two weeks of pregnancy.

Here’s a more in-depth answer to that perennial question of How many weeks pregnant am I?

What is a “due month?”

A “due month” is a more accurate timeframe for when you can expect to deliver your baby. Only 4% of babies are born on their due date. Whereas 80% of babies arrive either two weeks before the due date or two weeks after. Hence the term “due month.”

The length of a natural pregnancy can vary by as much as five weeks. (source)

A due month helps some mamas reduce the stress and fear of going past their due date.

To calculate your due month, simply subtract two weeks from your EDD given by your practitioner or our due date calculator and also add two weeks to your EDD. Voilà, your due month!

Yet another way to handle this tricky business of calculating your pregnancy calendar is to add two weeks to the end of your EDD and say, “Baby will be here before [that date].”

What is Naegele’s rule for due date calculation?

Naegele’s rule is what this due date calculator and pregnancy calendar is based on. Named after a German Obstetrician who practiced in the early 1800’s, Naegele’s rule predicts childbirth to occur 280 days after the first day of the last menstrual period.

However, Naegele’s rule assumes that your cycle is 28 days long with ovulation occurring on day 14, which isn’t the case for many women. So other ways of calculating your due date may be more accurate.

(How accurate is your due date? Find out in this post.)

Modern data suggests that women have their babies a few days after their due date on average. Studies like this one found that Naegele’s rule consistently places the due date about 2-4 days too early. So a better estimate may be 40 weeks and 3 days from LMP.

Alternatively, you can use our Advanced Due Date Calculator, which uses the Mittendorf-Williams rule to calculate your due date, which has been shown to be more accurate.

What’s the Mittendorf-Williams rule?

This study done in 1990 showed that pregnancy lasted an average of 288 days past LMP for Caucasian first-time moms. For Caucasian women who were not first-time moms, their date of delivery averaged 283 days past LMP (3 days after Naegele’s rule predicted). This finding is known as the Mittendorf-Williams rule.

While Naegele’s rule is still the most widely used formula for a due date calculator, the Mittendorf-Williams rule is proving to be more accurate. But it’s a much more complex calculation, taking into account:

• Maternal age
• Race
• Height
• Weight
• Number of pregnancies
• Average luteal phase length
• Maternal education
• Alcohol during pregnancy
• Coffee during pregnancy

Our Advanced Due Date Calculator uses the Mittendorf-Williams rule.