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Due Date Calculator: How do you calculate due date?

So you’ve gotten your BFP (big fat positive) 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 and discover the best estimate for when your little bundle of joy will make his or her appearance.

How do due date calculators work?

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

Most online due date calculators, including ours, use 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.

What is Naegele’s rule for due date calculation?

Naegele’s rule is what most due date calculators and pregnancy calendars are 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 a due date that may be more accurate.

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 try the Mittendorf-Williams rule to calculate your due date, as described below.

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 for calculating pregnancy due dates, 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.

What is an estimated due date?

An estimated due date (EDD) is a “best guess” as to when baby might be born based on a conception calculator. 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 estimated due date. (See “due month” section below.)

How are the weeks of pregnancy calculated?

The pregnancy calculator estimates your due date from the first day of your last menstrual period. If you have a 28 day cycle and discover you are pregnant on the day you should have had your period, you are already considered 4 weeks pregnant. This means that you have completed your 4th week of pregnancy (even though you conceived just two weeks earlier. I know, confusing). The following day, you are considered to be four weeks one day pregnant.

How do you figure out how many weeks pregnant you are?

In short, use our due date calculator above! But if you were calculating your baby’s due date manually, it goes a little something like this:

  • Determine the first day after your last menstrual period
  • Add 280 days to that date

However, the average length of your cycle can modify this.

  • 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

You can also calculate how many weeks pregnant you are by the date you conceived or the date you had sex. More info below.

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. Mamas who track their ovulation may know their exact date of conception. But for the rest of us, date of conception can be tricky to pinpoint. After all, sperm can live in the 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.

How do you calculate due date by date of intercourse?

Sperm can live in the vagina for up to five days. That means if you have sex on Monday, you may conceive on Monday, Tuesday, Wednesday, Thursday, Friday, or even Saturday. To come up with a date of conception based on when you had intercourse, add two days to the date when you had intercourse. This isn’t an exact figure, but it’s a fair average.

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 likely be here before [that date].”

What is “gestational age?”

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). The due date calculator helps you figure this out. However, as we said earlier, gestational age is counted from the first day of the LMP. Therefore, when baby is conceived (around 2 weeks after LMP) he is considered to be already 2 weeks gestational age. Conceptional age (CA) is the age of baby from when he was conceived.

What does GA stand for in an ultrasound?

If you get an ultrasound you may notice a “GA” on the image with a number of weeks and days. This stands for, you guessed it, gestational age. But 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 the real 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. It’s up to you to consider the pro and cons of ultrasounds. This study shows that early dating ultrasounds don’t change the incidence of induction.

What is a LMP date?

LMP simply stands for last menstrual period. The last menstrual period is the last period you had before becoming pregnant. Your LMP date is the first day of your last menstrual period. This date is used by the due date calculator to figure out your due date (by adding 40 weeks, per Naegele’s rule).

How are the trimesters of pregnancy divided?

Pregnancy is divided into three trimesters.

The first trimester, though similar in duration as the other two, feels much shorter because for the first four plus weeks you don’t know you’re pregnant!

The second trimester is often a nice break from the nausea, fatigue, and, well, just feeling bloated versus actually pregnant! Many women find the second trimester to be the most comfortable, which is why it’s coined “The Magic Middle.”

Though you’ll likely start showing in the second trimester (if not earlier), the third trimester is when you’ll become obviously pregnant, and you may begin to feel uncomfortable. But all of the third trimester discomfort is designed perfectly because, by the end, you’re more than ready to go through all the stages of labor and deliver your bundle of joy!

Related Resources

Ready to calculate your due date?

First day of your last period:

Average cycle:


Date you conceived:

Already know your due date? Click here