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When you learn you are expecting, you’re naturally going to have many questions. Besides the gender of your baby, one of the most common is: “When is the baby due?”

Assigning a due date gives you something to look forward to and plan for, but this isn’t an exact science. Instead, it is your obstetrician’s or midwife’s best guess regarding how long your baby will gestate, or stay in your womb.

Technically, you become pregnant when an egg, which has already been fertilized by a sperm (usually in the fallopian tube), implants in the wall of your uterus. However, for most women, there is a very small window of actual fertility, and it can be impossible to know when fertilization actually occurred.

What is less difficult to determine, however, is the first day of your last menstrual period.

Because you typically remember this day, your physician will use the date of your last period to determine your due date. Doctors will typically calculate your due date as 40 weeks, or 280 days, from your last menstrual period. For example, if your last menstrual period was May 1, then your due date will be February 5.

This usually means for the first week or two of your 40-week pregnancy, you weren’t pregnant at all!

While this is the most common method used to determine due date, in the early 1800s, a physician named Dr. Franz Karl Naegele developed a simple method to determine due date. It’s now known as the Naegele rule:

  • Step 1: Determine the first day of your last menstrual period.
  • Step 2: Add one year to this date.
  • Step 3: Subtract three months.
  • Step 4: Add seven days.

While this calculation can give you some idea of when you could expect to deliver, only an estimated 5 percent of babies are born on their due date, according to the American Pregnancy Association.

Considerations for irregular periods

While Naegele’s rule can be a jumping-off point for determining due date, the truth is that many women do not have regular menstrual periods. If this is the case for you, your healthcare provider might recommend an ultrasound. This imaging method allows your doctor to visualize your baby and look for signs of growth that can indicate your baby’s age.

Due to the fact that ultrasound may help with dating, the earlier that the ultrasound is done will help with a more accurate date. Of course, babies can develop at varying rates, which is why your physician will continue to monitor development and may adjust the due date occasionally throughout your pregnancy. Due dates will typically not change once it has been established early in pregnancy. When the last menstrual period is not known or the ultrasound is done later in pregnancy, the due date may change when prenatal care is established.

Takeaways

  • Medical providers typically calculate your due date to be around 40 weeks from the first day of your last menstrual period.
  • Naegele’s rule is a commonly used method for calculating an estimated due date.
  • Women who have irregular periods may benefit from regular ultrasounds to determine growth, which can indicate a due date.

References

  1. American Pregnancy Association. Calculating Conception.
  2. March of Dimes. Calculating Your Due Date.
  3. Medscape. Issues in Pregnancy Dating: Revisiting the Evidence.
  4. What to Expect. How to Calculate Your Due Date.

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