Calculate Your Due Date and How Many Weeks Pregnant You Are

Learn how a due date calculator determines your due date

“When’s your due date?” Friends, family, and health care providers will ask you this question numerous times during your pregnancy. It’s also likely one of the first bits of information you’ll seek out after you find out you’re pregnant. Friends and family will want to know so they can look forward to greeting your new baby (and supporting you, the new mother). Doctors, midwives, and nurses will want to know so they can track important health milestones and make decisions about interventions and prenatal testing.

With so much focus and excitement over an assigned day, you’d think it’d be an accurate number. But it’s not! Only 5 out of 100 women will have their baby on their actual due date.

Still, determining which week of pregnancy you’re in is crucial to receiving good prenatal care and planning for your upcoming life change. A due date calculator can help. Take a minute to learn your due date and then read about everything you need to know.

Why Due Date Is Important

Your due date—sometimes also referred to as the estimated date of delivery—is less of a deadline and more like a time marker. Your due date indicates the 40th week since your last menstrual period or the 38th week from ovulation. (These are not always the exact same date since not every woman ovulates on day 14 of their cycle. A woman who ovulates later may truly be due 41 weeks after her last menstrual period. Her due date based on ovulation, and not based on her last menstrual period, is the more accurate indicator.)

As mentioned above, your odds of giving birth on your due date are very small. You are, however, likely to give birth within a four-week period that surrounds your due date. In other words, about two weeks prior to and two weeks after your assigned due date. About 70 out of every 100 women will have their baby within 10 days of their due date. Assuming there are no complications, others will have their baby either a little earlier or a little later than that.

If your due date is so inaccurate, then why is it important?

While the specific day doesn’t mean much, it does give you an idea of where you are in the pregnancy as far as weeks are concerned. This information is used to:

Track fetal development. Your healthcare provider considers which week of pregnancy you are in to determine if the pregnancy and fetal development are going well or if there might be a problem. When should a heartbeat appear on a transvaginal ultrasound? When should a fetal heart tone be detectable with a handheld Doppler? When should the baby start moving? What should the fundal height be? (More on fundal height below.)

All of these are determined by which week of pregnancy you’re in.

Schedule appropriate pregnancy tests. Some prenatal tests need to be conducted during a particular time frame. For example, an ultrasound to determine viability and look for a heartbeat shouldn’t be done before six weeks. An AFP blood test—used as a screening for birth defects, a possibly miscalculated due date, or other pregnancy irregularities—should ideally occur between week 16 and week 18. An ultrasound to determine whether twins are sharing a placenta and/or amniotic sac needs to occur between week 11 and week 14 of the pregnancy.

Determine whether labor has started too early. Babies who are born too early are at risk for numerous health problems. There are steps we can take to slow or halt preterm labor (when it’s caught in time.)

That said, there are also risks to the medications used to stop preterm labor, both to the mother and the baby. You and your doctor have to decide together when the risks of allowing labor to continue outweigh the risks of intervention. This is partially determined by knowing which week of pregnancy you are in.

Determine if the baby may be post-dates. There are also risks to the baby and mother if a pregnancy goes on too long. A post-term pregnancy is when birth doesn’t occur by week 42, or 14 days after the estimated date of delivery. When this happens, your midwife or doctor may decide to induce labor.

Provide an idea of when you might expect to have your baby. There’s a lot of planning that goes into having a baby! It helps to have an idea of when you might expect to go into labor and when you might expect to be taking care of a tiny new bundle of joy.

How It's Calculated

The due date your doctor provides for you may be slightly different than the one you get using a fertility app. Your doctor may shift your due date after an early ultrasound. How is your due date calculated?

Your due date may be determined by...

  • The date of your last menstrual period
  • The date of conception (or ovulation), if you know it
  • Measurements taken during an ultrasound
  • The date of embryo transfer and age of embryo at transfer (if you had IVF)

Your due date is considered to be 40 weeks (or 280 days) after your last menstrual period, or 38 weeks (or 266 days) after ovulation. If an early ultrasound is used to determine or shift your due date, it’s still based on this basic idea of a 280-day gestational period. There’s some debate in the scientific community on whether the 280 days is really the most accurate gestational length, but currently, this is the most commonly used timeframe.

Assuming you know the date of your last menstrual period, your primary care provider will use this date to figure out your due date. Many midwives and obstetricians use a pregnancy wheel, which is a simple tool that quickly gives the 280th day from your last period.

There is a formula for determining the 280th day past your last menstrual period, known as Naegele's rule. This is what’s used for the due date calculator on this page and is also how a pregnancy wheel works. You can calculate the date yourself, too.

Naegele’s rule works like this:

  • Take the date of your last menstrual period
  • Add one year
  • Add seven days
  • Move the date back three months

This will give you your due date.

Here is an example:

  • If your last menstrual period was August 11th, 2018…
  • Add one year: August 11th, 2019
  • Add seven days: August 18th, 2019
  • Move the date back three months: May 18th, 2019
  • Your due date would be May 18th, 2019

What if a fertility app you’ve used gives a slightly different date than the one calculated by your doctor? Your fertility app is likely looking at your estimated ovulation date and not the date of your last period. If you ovulated earlier or later than day 14 of your cycle, this would shift your due date accordingly.

Your due date in relation to ovulation is considered to be more accurate than one calculated based on your last menstrual period. If you know when you ovulated, or you know your cycles are longer than average, share that information with your doctor. The dates may be just a week apart, but that week can make a big difference. For example, if you’re having a scheduled C-section, you wouldn’t want to schedule it too early. Or, if your doctor is trying to decide whether your pregnancy has gone too far past your due date, you might want to wait another week before considering induction.

Note: What if you don’t know when your last period was? If you’re unsure, you’re far from alone. Research has found that just 1 in 3 feel certain they know the date of their last period. If this is your situation, your doctor may order an early ultrasound.

Using Ultrasound

An ultrasound in the first trimester of pregnancy—prior to 13 weeks—can be used to provide an estimated date of delivery or confirm a due date determined by your last period. Not every expectant mother will have an early ultrasound. If you don’t know when your last period was or there are any questions as to how far along you are, your doctor may order an ultrasound to date the pregnancy.

A transvaginal ultrasound is used for early pregnancy. Getting a good visual of the fetus is difficult in early pregnancy with a trans-abdominal wand. A transvaginal ultrasound is a slender wand that the ultrasound technician places a condom over, some lubricant jelly, and then will likely hand over to you to insert into your vagina. The technician will then take the handle of the transvaginal ultrasound wand and continue the imaging.

During the ultrasound, the technician will measure the fetus’s length from “crown to rump.” This is how they estimate how many weeks old the fetus is, and from that, provide an estimated due date. Ideally, this should be done between 9 weeks and 13 weeks (based on your last menstrual period.)

What if an ultrasound couldn’t be conducted before 13 weeks? There is an alternative: biparietal diameter (BPD) ultrasound. Some studies have found this to be as accurate as crown-to-rump measurements in the first trimester.

One study compared how often a pregnancy went past dates when the due date was calculated using last menstrual period, crown-to-rump measurements, or biparietal diameter. They found that...

  • 31 percent went past 41 weeks when using last menstrual period
  • About 25 percent went past 41 weeks when using crown-to-rump measurements
  • Only about 17 percent went past 41 weeks when using biparietal diameter

All that said, ultrasound due dates are still not perfect. The position of your uterus or the technician’s skill can all cause variations in measurements. Also, we don’t really know if all fetuses grow at the exact same rate, even in early pregnancy. Your due date determined by ultrasound is still only a guess-estimate and not a written-in-stone deadline.

Adjustments and Changes

Your due date is only as accurate as the information available to you and your doctor. So, if you aren’t sure when your last menstrual period was, figuring out your due date may be tricky. Even if you know your last period, if you ovulated later or earlier than average, this can change your actual due date. These are all reasons why your due date may be adjusted after an early ultrasound.

If the ultrasound due date is different from the date determined by your last period, both dates should be noted in your medical records. If the estimated ultrasound due date is less than seven days different from the last menstrual cycle due date, your due date doesn’t change. If it’s off by more than seven days, then your due date may be adjusted.

Note: If you had IVF treatment, your due date will never change. IVF due dates aren’t determined by your last period or the date of conception (which would be egg retrieval day.) They are determined by embryo transfer day and the age of the embryo at transfer. This is because the day an embryo implants itself into the uterine lining affects the length of gestation.

Fundal Height

Your midwife or doctor may measure your fundal height at your pregnancy well-checks. The fundal height is the measurement in centimeters from your pubic bone to the top of your uterus. It should grow at a predictable rate as your pregnancy continues.

After you reach 20 weeks, your fundal height will typically be the same as the number of weeks pregnant you are. In other words, at 21 weeks pregnant, your fundal height may be somewhere around 21 cm.

Sometimes, the fundal height doesn’t match how far along you are in the pregnancy. Slight variations are normal, but if you are measuring much smaller or larger than expected, your primary care provider may want to investigate. There are many reasons this may occur, some of them perfectly normal. Your doctor may schedule an ultrasound to be sure everything is okay.

However, your due date should not be adjusted based on your fundal height. It is not an accurate measure of gestational age.

Factors of Actual Birth Date

The majority of babies are born over a 35-day span: with birth occurring anywhere from about 37.5 weeks to 42.5 weeks. If you treated your work deadlines the way due dates worked, you’d probably lose your job!

As mentioned above, few women give birth on their due date. A due date is also not a deadline—in other words, it doesn’t mean you’ll give birth by that date either. You could have your baby a week later, and everything is still considered to be on time.

So, what determines when you’ll have your baby? Here are some theories.

Older moms tend towards longer pregnancies. Research has found that for every year of age, the pregnancy is likely to be about one day longer. A 32-year-old mother is likely to give birth, on average, ten days later than a 22-year-old mother.

First-time moms tend to go into labor a little later. You’re more likely to go a few days past your due date if this is your first baby.

If you were a big baby at birth, your pregnancy might go a little longer. It’s unclear whether a mother’s current weight determines when she’ll go into labor. However, how much she weighed at birth does seem to affect the length of her pregnancy. Mom’s who were bigger babies may have slightly longer pregnancies themselves.

Your pregnancies are likely to all be about the same length. If you’ve already had two kids, and they were born around 41 weeks, your third kid is likely to also come at around 41 weeks.

About 1 in 10 pregnancies will be premature. Some babies will come before 37 weeks, which is too early. There are a variety of risk factors that may cause you to go into labor too soon, including if you’re pregnant with twins or more, if you have a history of preterm labor, or if there are abnormalities with the cervix or uterus. A history of infertility may increase your risk of premature birth. Also, some lifestyle and poor health habits can increase the risk of premature labor.

Sharing Your Due Date

“Did you have the baby yet?” This may be the most annoying question you’ll get when you’re expecting, especially as you’re approaching your due date. Being asked this question over and over can make you more anxious about the upcoming birth.

For this reason, some women choose not to share their due date. Instead, they offer a very vague answer. “Sometime in July,” they may say. Or they give the very latest possible date, like two weeks past their estimated due date.

There are no right or wrong decisions when it comes to sharing details about your pregnancy, of course. Whether you decide to tell people the due date, keep it completely secret, or go vague on the details, it’s your choice.

Measuring Pregnancy Length

Pregnancy can be measured in days, weeks, months, or trimesters. The months aren’t all four weeks long, some are more like five weeks. Understanding how they fit together can be confusing! Here’s an easy to understand guide.

First trimester

Your first trimester is considered to be week 1 through week 13. When it’s broken down into months, it looks like this:

Second trimester

Your second trimester is week 14 through week 27. When it’s broken down into months, it looks like this:

Third trimester

Your third trimester is week 28 until birth. When it’s broken down into months, it looks like this:

While the third trimester ends officially at 40 weeks, you’re not technically overdue until you reach week 42. So your “last month” of pregnancy can end up being seven weeks long!

Normal Length of Pregnancy

What is considered a full-term pregnancy? As mentioned, while 40 weeks is the average, it’s not unusual to deliver your baby earlier or later.

In the past, 37 weeks was considered to be a full-term pregnancy. This definition of full-term carried with it implications on when it was considered safe to induce labor or schedule a cesarean section. However, we now know that being born at 37 weeks can increase some health risks. It’s not nearly as risky as being born before 37 weeks, but it’s not ideal either.

For this reason, the American College of Obstetricians and Gynecologists has created new terms: early term, full term, late term, post-term. Here’s how they break down:

  • Early term is 37 weeks through 38 weeks and six days
  • Full term is 39 weeks through 40 weeks and six days
  • Late term is 41 weeks through 41 weeks and six days
  • Postterm is any pregnancy that goes beyond 42 weeks

Sometimes, when talking about twins or triplets, you’ll hear people say things like, “Twins are full-term at 37 weeks.” But this isn’t truly accurate. All pregnancies—including twins, triplets, or more—aren’t truly full-term until they reach 39 weeks. It’s just that you’re more likely to go into labor earlier, and if you do make it to 37 weeks with twins, that’s pretty good!

A Word From Verywell

Your due date helps you and your healthcare providers track your pregnancy and plan for birth. It’s an important date, but it’s not a deadline. You may have your baby before or after your due date, and that’s okay. If your due date comes and goes, you might start to wonder if you’re going to be “pregnant forever!” Don’t worry. While those last days can feel like an eternity, remember that your baby will arrive when it’s ready for the world and not a minute earlier. To be sure the baby isn’t brought into the world too soon—as long as everything otherwise looks fine—the American College of Obstetricians and Gynecologists recommends that induction of labor not be considered until week 41 begins. (That’s seven days past your official due date.) The wait is worth it.

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