Length of pregnancy and due date

After a positive pregnancy test, the question on many women’s minds is: what’s my date of delivery? Determining a specific date, however, is more complicated than you might think. Here’s how to calculate your due date.

How long is a pregnancy?

Specialists estimate that a pregnancy generally lasts 40 weeks (280 days). However, some studies now suggest that a normal pregnancy may actually last a little longer. About 50 percent of first-time moms give birth after 40 weeks and 5 days (285 days). For a second pregnancy, delivery usually occurs around 40 weeks and 3 days.

Some women may also have a longer pregnancy: the same study revealed that about 25 percent of women having their first baby give birth after 41 weeks and 2 days (289 days). In theory, however, no pregnancy will exceed 42 weeks and 6 days (300 days). Doctors will usually offer to induce labour between weeks 41 and 42. By then, it’s usually safer to induce labour than to wait.

How do I calculate my due date?

During your prenatal checkup, your health care provider will try to determine your expected date of delivery. Your due date is generally estimated to be 40 weeks—280 days—after the first day of your last menstrual period.

Your doctor or midwife will therefore include the 2 weeks after your last period in the number of weeks of pregnancy, even if you weren’t pregnant yet. In other words, being 20 weeks pregnant means that 20 full weeks have passed since the first day of your last period.

How reliable is the estimated delivery date?

Although this method of calculating due dates has been used for over 200 years, it’s criticized by many health professionals.

• First, Naegele’s rule is based on the assumption that a menstrual cycle lasts 28 days. However, many women have irregular cycles. In fact, 30 percent of women report having a cycle of at least 30 days.
• The rule assumes that ovulation occurs exactly on day 14 of a 28-day cycle, which is not the case for most women. In the general population, ovulation tends to occur in the second half of the menstrual cycle. In women with a 28-day cycle, only 10 percent ovulate on day 14, and only 30 percent of these women are fertile between days 10 and 17.
• Many women do not remember the date of their last period. According to one 1985 study, 79 percent of women are certain of their last menstrual period, meaning they can determine the week in which it occurred.
• Naegele’s rule doesn’t take into account the fact that some embryos take longer to implant in the uterine wall, thus prolonging the pregnancy.
• Furthermore, intermenstrual bleeding or implantation bleeding can be mistaken for a light menstrual period.

The date of conception is therefore likely to provide more accurate results than the date of your last menstrual period. However, most women don’t know when they’re ovulating. Usually, basal body temperature methods and ovulation tests are necessary to detect ovulation.

For this reason, many professionals believe it’s impossible to predict a woman’s exact due date. Only 4 percent of women give birth on their estimated date of delivery. Soon-to-be moms should therefore focus on their expected week—or even month—of delivery.

The use of ultrasounds

An early pregnancy ultrasound has been shown to be more accurate than the date of your last menstrual period for determining your expected date of delivery. This is especially true if the ultrasound is performed after 8 to 16 weeks of pregnancy. During the scan, a technician measures the baby’s height, from the crown of their head to their buttocks. Since babies develop very similarly in early pregnancy, this number can be used to determine the length of your pregnancy. However, the accuracy of the measurement can vary depending on the ultrasound.

If the date calculated using Naegele’s rule differs from the ultrasound date by more than 7 days, your doctor may choose to change the expected due date to reflect the first-trimester ultrasound result. If the ultrasound is performed after 12 to 20 weeks of pregnancy, however, the difference must be more than 10 days for your doctor to consider changing the date.

 Things to keep in mind

• A pregnancy generally lasts 40 weeks (280 days).
• The length of pregnancy can vary from one woman to another.
• It’s difficult to determine an exact due date.

Scientific review: Amélie Guay, M.Sc., PNC(C), perinatal advanced practice clinical nurse, CHUM Research and copywriting: The Naître et grandir team Updated: September 2020

Photo: iStock/aldegonde