How Accurate are Pregnancy Due Dates?

Upon finding out that you’re pregnant, the next question is usually “How far along am I?” Luckily, there are many helpful tools that exist to help you determine your Estimated Due Date (EDD), each with its own margin of error. Here’s a bit more information on both medical and non-medical “calculators” that are available to you.

It is important to note that birth usually occurs between 37 and 42 weeks. In addition, fewer than 5% of births occur exactly on the 40-week mark; 50% of births occur within one week of the EDD (between 39-41 weeks) and 80% occur within 2 week of the EDD (between 38-42 weeks). Keep in mind as you read this blog that it is much more useful and accurate to refer to a range of due dates, instead of one specific day. (NOTE: The gestational age is calculated based on the date of conception, which is typically 14 days after a woman’s last menstrual period. Using this date, birth occurs 38 weeks after conception.)


Naegele’s Rule: Franz Naegele was a German Obstetrician that developed the rule for estimating the date of delivery around 1830. According to his rule, start with the first day of your last menstrual period, add 7 days and subtract 3 months. The cycle will be approximately 280 days (40 weeks) from the last menstrual period. (Naegele’s rule assumes a menstrual cycle of 28 days, and so EDD will be later if cycle is longer, etc.)

Parikh’s Formula: This calculation is similar to Naegele’s Rule, however it accounts for the duration of individual cycles, and does not assume a 28-day cycle. It recommends adding 9 months to the first day of your last menstrual cycle, subtracting 21 days and finally, adding the duration of previous cycles (ex. 31 days).

There are many Pregnancy Calculators that exist online that will take your cycle length into account:


Fundal Height: The fundal height, or McDonald’s Rule, is a measurement of the size of the mother’s uterus. The measurement helps assess the growth and development of the fetus is measured from the top of the pubic bone to the top of the uterus. The length, in centimeters, should match the growing baby’s gestational age, to an accuracy of 1-3 cm (ex. A measurement of 20 cm would be seen on a 17-23 week pregnant mother.) This measurement is most often used following the 20-week mark and is accurate within 4-6 weeks.

Fetal Heart Rate: The fetal heart rate can be measured beginning at 10-12 weeks gestation electronically. This method is accurate within 4-6 weeks. At 20 weeks, the fetal heart tones are usually heard with non-electric devices (fetoscope).

Bi-Manual Exam: During the first trimester, a manual exam will be conducted and is accurate to within 1-2 weeks. By feeling for the uterus, it can be determined how far along a pregnant mother is. At 12 weeks, the uterus should be felt above the pubic bone. At 16 weeks, the uterus should be at the belly button and the patient will be able to feel fetal movement. Ultrasound: Ultrasounds seem to be the most accurate early in the pregnancy with an error margin of 5-7 days early on. If an ultrasound is done later in the second trimester or even the third trimester, delivery date estimations may be off by up to a few weeks.

Always remember to enjoy your pregnancy and try not to worry too much about exact dates. Your body will know when it’s ready!

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