Irregular period due date calculator

Due Date Calculator — Amazingly Accurate Pregnancy Calculator

First day of your

Please choose the day your most recent period started.

Average cycle: 20 days21 days22 days23 days24 days25 days26 days27 days28 days29 days30 days31 days32 days33 days34 days35 days36 days37 days38 days39 days40 days41 days42 days43 days44 days45 days

or

Date you

Conception is approximately two days after intercourse.

Already know your due date? Click here

Pregnancy Week by Week Universal Header

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).

Due date calculator quote Our standard due date calculator adds 280 days (40 weeks) to the date of your last menstrual period (LMP)

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

What is the date of conception due date calculator illustration

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

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.)

Baby due date on a calendar with pregnant woman background – only 4% of babies are born on their due date

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

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.

Due date calculator quote 2 Modern data suggests that Naegele’s rule places the due date about 2-4 days too early

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.

Related Resources

• Our FREE Natural Pregnancy Week-By-Week Series ?
• My Bestselling Natural Pregnancy Guide ?
• Reverse Due Date Calculator ◀️
• Natural Birth Stories ??
• Baby Name Finder ?

Ready to calculate your due date?

First day of your

Please choose the day your most recent period started.

Average cycle: 20 days21 days22 days23 days24 days25 days26 days27 days28 days29 days30 days31 days32 days33 days34 days35 days36 days37 days38 days39 days40 days41 days42 days43 days44 days45 days

or

Date you

Conception is approximately two days after intercourse.

Already know your due date? Click here

Mittendorf-Williams Due Date Calculator — Most Accurate Pregnancy Calculator

Reviewed by Maura Winkler, CNM, IBCLC

Our advanced calculator utilizes the Mittendorf-Williams rule, which factors in more info about you to provide more accurate results.

When was your

Please choose the FIRST day your period started.

Average cycle: 20 days21 days22 days23 days24 days25 days26 days27 days28 days29 days30 days31 days32 days33 days34 days35 days36 days37 days38 days39 days40 days41 days42 days43 days44 days45 days

How long is your

The number of days between ovulation and your next period.

10 days11 days12 days13 days14 days15 days16 days17 days18 days19 days20 days21 days22 days23 days24 days25 days26 days27 days28 days29 days30 days

What’s your

Enter your pre-pregnancy weight. People with higher BMIs usually carry longer.

• Imperial
• Metric

feet   inches

lbs

How

Younger people tend to have longer pregnancies.

years

What’s your

Caucasians tend to deliver later.

Caucasian

Non-caucasian

Did you go to

People who attended college carry a bit longer. (Weird, right?)

Yes, I went to college

No, I didn’t go to college

Do you drink

People who drink usually carry an extra day.

Yes, I drink alcohol

No, I don’t drink

Do you drink

People who drink a lot of coffee deliver earlier.

cups/day

Is this your

First pregnancies last about 3 extra days longer.

Yes, it’s my first pregnancy

No, I’ve been pregnant before

Have you had any

Bleeding, especially later, can indicate an earlier delivery.

1st trimester

2nd trimester

3rd trimester

Please select

Each of these factors may cause a shorter pregnancy.

History of miscarriage

History of stillbirth

Incompetent cervix

Placenta Previa

Placenta Abruption

Premature rupture of membranes

Pregnancy-induced hypertension

This advanced due date calculator utilizes the Mittendorf-Williams rule, which factors in more info about you and your pregnancy to provide more accurate results.

What is the Mittendorf-Williams Rule?

Named for Robert Mittendorf, assistant professor and director of health studies in obstetrics and gynecology, and Michelle Williams, an epidemiologist at the University of Washington, the Mittendorf-Williams Rule uses 16 significant factors—maternal age, pre-pregnancy weight, race, college education, alcohol and coffee use, hypertension, and other medical conditions—that provide a more accurate way of dating pregnancy and identifying whether a woman is at risk for preterm delivery. (source)

The Mittendorf-Williams rule is based on two studies:

• This study done in 1990 that showed pregnancy lasted an average of 288 days past a woman’s last menstrual period (LMP) for Caucasian first-time moms. For Caucasian women who were not first-time moms, their date of delivery averaged 283 days past LMP (three days after Naegele’s rule predicted).
• The second study done in 1993 suggests that there are several factors, including number of previous births, age, and race that influence the length of pregnancy.

Until now, there has been no successful way to determine numerically the risk for preterm delivery. By identifying these women, we may be able to lower the infant mortality rate by intensifying a woman’s prenatal care, and medically manage patients differently based on this new information. — Robert Mittendorf

Though few women give birth on their actual due date, a prospective study comparing the Mittendorf-Williams Rule to Naegele’s rule found the Mittendorf-Williams Rule to be twice as accurate in predicting the length of gestation. (source)

Advanced Due Date Calculator vs. Basic Due Date Calculator

• If you know how long your luteal phase is, you can use our advanced due date calculator above, which is based on the Mittendorf-Williams rule, to get a more accurate due date.
• If you don’t have this information handy, use our more basic due date calculator, which follows “Naegele’s rule” and predicts childbirth to occur 280 days after the first day of your last menstrual period.

In other words, though Naegele’s rule is still the most widely used for calculating pregnancy due dates, the Mittendorf-Williams Rule and our advanced due date calculator will provide the most accurate answer to the burning question of, “how many weeks pregnant am I?”

Still, it’s important to remember, no matter what type of tool you use, a due date is only ever just an estimate. The reality is very few women actually give birth on their due date, no matter how accurate the due date calculator they’ve used is.

Read more about how pregnancy due dates are calculated (and why they are hardly ever set in stone) on our basic due date calculator page.

When Did I Conceive?

If you already know your due date and wish to know when you most likely conceived, try our reverse due date calculator. As long as you know your due date (or your or your baby’s birthday), the tool will calculate the estimated conception date and intercourse dates. Pretty cool, huh?!

Online calculator and monthly calendar

The tool allows you to calculate the dates of menstrual cycles and visually display them on the monthly calendar for 1 year ahead.

This way you can find out the days of ovulation, the high probability of conceiving a child and the dates of safe days.

Calculator

Calendar

Period calculator

Cycle time: 0037 52 26 27 28 29 30 31 1 01 2 3 4 5 6 7 8 02 9 10 11 12 13 14 15 16 17 17 17 17 17 17 17 17 17 17 17 17 0068 20 21 22 04 23 24 25 26 27 28 29 05 30 31 1 2 3 4 5