How To Calculate Your Pregnancy Due Date

Pregnant woman with hourglass

When you start announcing your pregnancy, everyone will start asking what your due date is. While this date can help you plan your life around your birth, there is no precise way to tell for sure what day your baby will be born. Every pregnancy's due date carries a plus or minus two weeks. In fact, less than four percent of women actually deliver on their due dates. 

Over twelve percent of babies are born prematurely or before 37 weeks. Some pregnancies actually last longer than 42 weeks, though this is becoming increasingly rare. The average length of pregnancy is 280 days or forty weeks and there are some factors that can contribute to your due date. These factors include: ethnicity, prenatal care, prior pregnancies, prenatal nutrition and mother's age.

  1. Naegele's Rule. The Naegele's rule is the one your care provider will use to determine your due date. For this method to work, it is assumed you have a 28 day cycle and you ovulated on day 14 of your cycle. For the Naegele's rule, you take the first day of your last period, and subtract three months. Then you add one year and seven days to the date. This date will equal your 'due date' according to the Naegele's rule. This method works well, but only if your cycle is 28 days. Many women do not have 28 day cycles, or they have irregular cycles.
  2. Mittendorf-Williams Rule. In a study done by Mittendorf in 1990, it was discovered that on average, it takes 288 days from the first day of your last period to delivery in Caucasian women without prior deliveries. Using this method, you will want to take the first day of your last period and subtract three months. Then you add fifteen days if this is your first baby and you are Caucasian. If you are not Caucasian or this is not your first baby, you will want to add on ten days. Some studies have found the Mittendorf-Williams Rule to be more accurate than Naegele's Rule in calculating due date.
  3. Websites. Many baby websites offer due date calculators that can help you calculate you due date. For the majority of them, you will need to know the date of your last menstrual period and may need to know the average length of your cycle. Your care provider will also let you know what your due date is.
  4. Ultrasounds. Ultrasounds can give you a general idea for your due date. Your ultrasound technician will measure the baby's body parts, like head circumference and length of bones. Using these, the technician will be able to estimate your baby's gestational age, and thus your due date. 

 

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