The Best Way to Calculate Your Pregnancy Due Date

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The due date is the basis for many things you will need to know during your pregnancy. This date will help your doctor or midwife calculate how well your baby is growing and to know what tests should be offered when. And, of course, you may be anxious to learn your due date because it will give you an idea of when your baby may be born.

While only about three to four percent of babies are born on their due date, the due date gives you a general idea of when your baby will arrive. The two weeks on each side of your due date create the four-week due date window or “due month.” Some women prefer to give this information out, rather than a specific date, to help well-meaning friends and family understand that the due date is not a date to circle in red like an appointment on the calendar. There is plenty of variation in when your baby may show up.

How a due date is calculated

The due date, in its most basic form, is calculated based on your menstrual period. Using the first day of your last normal period, your doctor or midwife will calculate your due date using a formula called Naegele's Rule. This is how the rule works:

  • Write down the first day of your last period.
  • Add one year.
  • Subtract three months.
  • Add seven days.

For example, if the first day of your last period was April 2, 2020, your calculation would look like this:

  • April 2, 2020 plus one year is April 2, 2021.
  • April 2, 2021 minus three months is January 2, 2021.
  • January 2, 2021 plus seven days is January 9, 2021.
  • January 9, 2021 would be your due date.

The good news is that calculating your due date using Naegele's Rule is very simple given all of the apps and calendars available online. Your doctor or midwife may also have a circular calendar based on this rule called a gestation wheel that can be used to calculate your due date without doing the math.

Why Naegele's Rule isn't the gold standard anymore

As technology has developed, the concept of using a due date based on something as inflexible as the first day of your last period has been questioned, and Naegele’s Rule has taken a back seat. There are also women who cannot use the rule because they are not sure when they conceived. This has led to the use of other methods to calculate the due date, which are often cited as more accurate.

Ultrasound in the first trimester to calculate the due date

More and more practitioners are using an early ultrasound as their preferred way to date a pregnancy. Because the growth of the baby in the first trimester is fairly standard, even despite wide variation in genetics, it is relatively easy to predict a more accurate due date, without the concern of cycle variations or the inaccuracy of remembering when you had your last period. This method also works no matter how you conceived.

For this method to work, you must be receiving prenatal care in the first trimester and you must have an early ultrasound. Some practitioners do not do a standard early ultrasound. It's not imperative for most women but is an option they can discuss with their midwife or doctor. This method is considered the most accurate way to predict a due date.

Using Naegele’s Rule, a woman is told that her baby will likely be born in a 14-day window surrounding the due date. Using the ultrasound method, that window can be narrowed down to seven to 10 days, depending on how early the ultrasound is performed. However, it is important to keep in mind that these are still estimates.

Ultrasounds done after the first trimester yield a window closer to the 14-day standard of the cycle or calendar method using Naegele's Rule. It is also important to know that after the midway mark in pregnancy, ultrasound should not be used to predict the due date at all — after the midway mark, fetal growth is very different from person to person based on genetics.

When Naegele's Rule doesn't work

Naegele's Rule has been around for more than a century. While pregnancy hasn't really changed in that time, what has changed is our understanding that individuals have different bodies and may have different circumstances. For example, the rule may not work as well for women with shorter- or longer-than-average cycles. This could lead to an unnecessary induction of labor if it is thought that the baby is in danger because the pregnancy has gone on too long.

Another group that may need a more nuanced look at its due dates would be pregnant people who used certain forms of fertility help. For example, if you have had in vitro fertilization (IVF), you are calculating your pregnancy based on the date of your embryo transfer and not based on your period or cycle.

Points to remember about your due date

Your due date is calculated specifically for your pregnancy using one or more methods. The earlier you receive prenatal care, the more accurate your due date will be. You should consider using ultrasound in the first trimester to help you find your due date, particularly if:

  • you have a varying cycle length
  • you aren't sure when you got pregnant
  • you used certain fertility treatments

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