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If you’re expecting, naturally you want to know how to calculate your due date accurately so you can plan ahead. There are several methods for estimating due date, starting with the traditional, widely used one known as Naegele’s rule. Named for the German obstetrician Franz Karl Naegele, this method was published for midwives in 1830. 

Naegele wrote, “The usual calculation of the duration of pregnancy, namely, starting from the last menstruation, is correct in most instances; and conception within the last third of the cycle or in the second half between two periods is unusual and an exception to the rule.” Generations of midwives and doctors have learned and relied on Naegele’s rule, and it’s still used today. It’s based on the start date of your last period, adds a year, subtracts three months, and then adds seven days. 

How Accurate Is Due Date?

Naegele’s rule assumes a cycle is 28 days and ovulation occurs on day fourteen. Cycles longer or shorter than 28 days or any variability in ovulation can throw off the estimate. Therefore, this method is most accurate only if your period is like clockwork. Here are some interesting facts regarding estimated due dates:

  • An estimated 4–5% of women give birth on their exact due date
  • Most first-time moms give birth 3–5 days past their due date, and many carry their baby even longer
  • A 1994 study concluded that Naegele’s rule predicted the expected date of delivery three days too early in women with regular, known cycles

How Is Due Date Calculated? 

The American College of Obstetricians and Gynecologists (ACOG) recommends doctors use ultrasound measurement in the first trimester to establish an estimated due date. Today, it’s common for pregnant women to receive two different due dates — the first when the baby is the size of a pea on the initial ultrasound, and the second when she receives subsequent ultrasounds in the first trimester. 

How Are Pregnancy Weeks Calculated? 

For many centuries, it was accepted that the normal gestation period for humans was 10 lunar or nine calendar months. If you’re wondering how to count weeks of pregnancy, this once again goes back to Naegle’s rule. Count the number of days since your last period. For example, if it’s 84 days, congratulations, you made it to week 12 or the end of your first trimester! ACOG terms are based on the number of weeks pregnant:

  • Early term: 37 weeks to 38 weeks and 6/7 days
  • Full term: 39 weeks to 40 weeks and 6/7 days
  • Late term: 41 weeks to 42 weeks and 6/7 days
  • Post term: Longer than 42 weeks

How to Determine Due Date 

Other than ultrasound, the following methods can be used to determine your due date: 

  • Last menstrual period: Add 40 weeks or 280 days to this date
  • Date of ovulation (also considered conception date): Assuming you know it, add 266 days to this date. Ovulation is typically 14 days after the first day of your period, which is why this method is exactly 14 days less than the first method!

How to calculate if you’re an IVF Mom

  • IVF 3-Day Transfer Date: Add 263 days to the last day of your IVF transfer date
  • IVF 5-Day Transfer Date: Add 261 days to the last day of your IVF transfer date

While Due Date Isn’t a Certainty, Finding out your Baby’s Gender can be! 

Like most women’s periods, the development of human life doesn’t operate like precision clockwork. Estimating due date isn’t an exact science, and furthermore, pregnancy complications can change delivery dates. For example, an OBGYN may decide to induce labor to protect the mom and unborn child, significantly altering the timeline of the baby’s arrival. 

When you use Peekaboo, you can determine your baby’s gender as early as six weeks gestation! The Peekaboo At-Home Early Gender Test is easy, affordable, and accurate. That’s why it’s the only early baby gender reveal test endorsed by the American Pregnancy Association.

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