How Calculate Your Pregnancy Due Date

Your due date is 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.

There are a number of different ways to calculate your due date:

  • The date of your last period
  • The date of conception or ovulation
  • Measurements taken during an ultrasound
  • The date of implantation if you had in vitro fertilization (IVF)

With so much focus and excitement over an assigned day, you’d think it’d be an accurate number. But it’s not. The simple truth is that only 5 out of 100 women will deliver 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.

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.

Once your due date is calculated, you 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.

Around 70 out of every 100 women will have their baby within 10 days of their due date. Assuming there are no complications, others will deliver either a little earlier or a little later than that.

The due date is valuable for many reasons, including the preparation of your home and family for the new arrival. Other important reasons include:

Tracking 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. This includes:

  • When a heartbeat should appear on a transvaginal ultrasound
  • When a heart tone should be detectable with a handheld Doppler
  • When the baby should start moving
  • What the fundal height should be

Scheduling Tests

Some prenatal tests need to be conducted during a particular time frame. For example, an ultrasound to determine viability shouldn’t be done before six weeks. An AFP blood test—used as a screening for birth defects—should ideally occur between week 16 and week 18.

An ultrasound to determine whether twins are sharing a placenta or amniotic sac needs to occur between week 11 and week 14 of the pregnancy.

Determining If Labor Is Premature

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. With 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 to outweigh the risks of intervention. This is partially determined by knowing which week of pregnancy you are in.

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.

Calculating by Date

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.

Assuming you know the date of your last period, your primary care provider will use this to figure out the 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.

How to Use the Naegele's Rule

To get your due date based on your menstrual period:

  1. Take the date of your last period.
  2. Add one year.
  3. Add seven days.
  4. Move the date back three months.

By way of example, if your last period was August 11, 2019:

  1. Add one year: August 11, 2020
  2. Add seven days: August 18, 2020
  3. Move the date back three months: May 18, 2020
  4. Your due date would be May 18, 2020.

If fertility app gives you a different date than the one calculated by your doctor, it most likely because it was based on the 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.

Ovulation vs. Menstruation

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.

Using Ultrasound

If you’re unsure when your last period was, you’re not alone. Research has found that only one in three women feel certain they know the date of their last period. If this is your situation, your doctor may order an early 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 need one unless the date is uncertain.

A transvaginal ultrasound is usually performed between 9 weeks and 13 weeks of gestation. During the procedure, the technician will measure the fetus’ length from crown to rump. This is how they estimate how old the fetus is, and from that, provide an estimated due date.

Another method, called a biparietal diameter (BPD) ultrasound, measures the diameter of the baby's skull and may even be more accurate than the other methods. According to research:

  • Over 30 percent of pregnancies went past 41 weeks when using the Naegele's rule.
  • Around 25 percent went past 41 weeks when using the crown-to-rump measurement.
  • Only 17 percent went past 41 weeks when using the biparietal diameter.

With all that said, ultrasound due dates are still not perfect. 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 ultrasound due date is less than seven days different from the menstrual cycle due date, your due date wouldn't change. If it is off by more than seven days, your due date may be altered.

IVF Due Date

If you had IVF treatment, your due date will never change. IVF due dates aren’t determined by your last period, the date of conception, or even the egg retrieval day. They are determined by embryo transfer day and the age of the embryo at the time of the transfer.

With IVF, the due date is determined by the date that the embryo implants in the uterus. As the fertilization and implantation are both observed events, doctors are able to make more accurate calculations.

Role of 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 20 weeks, your fundal height in centimeters will typically be the same as the number of weeks you are pregnant. In other words, at 21 weeks pregnant, your fundal height should be around 21 centimeters (10 inches).

Sometimes, the fundal height won't match perfectly. Slight variations are normal, but if you are measuring much smaller or larger than expected, your primary care provider may want to investigate with another ultrasound.

While useful, the fundal height is not an accurate measurement of gestational age and would not affect the estimated due date in any way.

Normal Length of Pregnancy

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 carried implications on as two when it would be safe induce labor or schedule a cesarean section.

However, we now know that being born at 37 weeks can increase health risks. It’s not nearly as risky as being born before 37 weeks, but it’s not ideal either.

In response, the American College of Obstetricians and Gynecologists (ACOG) has created new definitions to describe the timing of labor:

ACOG Definition of Pregnancy Terms

  • 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 will 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.

While you may be at greater risk of earlier labor with twins, that doesn't affect how the timing of labor would be classified.

Factors That Affect Due 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. A due date is also not a deadline. You could have your baby a week later, and everything is still considered to be on time.

There are many factors, large and small, that can influence your due date. Among them:

  • Older mothers 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 mothers tend to go into labor later. You’re more likely to go a few days past your due date if this is your first baby.
  • Women often have their own predetermined schedules. Many women will tell you that their pregnancies were all about the same length. If you had two kids, each delivered around 41 weeks, your third kid is likely to arrive around 41 weeks.
  • Some women are at risk of preterm labor. All told, about one in 10 pregnancies will be premature. Risk factors include previous preterm birth, smoking, high blood pressure, diabetes, vaginal bleeding during pregnancy, and even periodontal disease.

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, your baby will arrive when it’s ready and not a minute earlier.

To be sure the baby isn’t brought into the world too soon, ACOG recommends that induction of labor not be considered until week 41 begins (or seven days past your official due date).

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