Scientific review: Amélie Guay, M.Sc., PNC(C), perinatal advanced practice clinical nurse, CHUM 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. 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. However, the accuracy of the measurement can vary depending on the ultrasound. Since babies develop very similarly in early pregnancy, this number can be used to determine the length of your pregnancy. During the scan, a technician measures the baby’s height, from the crown of their head to their buttocks. This is especially true if the ultrasound is performed after 8 to 16 weeks of pregnancy. If several ultrasounds are done between week 8 and the end of your first trimester, the due date established during the first ultrasound will take precedence.Īn 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.

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.įurthermore, intermenstrual bleeding or implantation bleeding can be mistaken for a light menstrual 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.

Many women do not remember the date of their last period.

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. In the general population, ovulation tends to occur in the second half of the menstrual cycle. The rule assumes that ovulation occurs exactly on day 14 of a 28-day cycle, which is not the case for most women. In fact, 30 percent of women report having a cycle of at least 30 days. However, many women have irregular cycles. How reliable is the estimated delivery date?Īlthough this method of calculating due dates has been used for over 200 years, it’s criticized by many health professionals.įirst, Naegele’s rule is based on the assumption that a menstrual cycle lasts 28 days. For example, if your last period was in May, your pregnancy is estimated to last 283 days, but if your last period was in early June, this number drops to 280 days. This can affect the calculated length of pregnancy.

However, adding 9 months is also problematic, as not all months have the same number of days. Naegele’s rule was therefore interpreted differently by physicians until the beginning of the 20th century, when American medical books specified that the 7 days should be counted from the first day of your last period. However, the rule doesn’t specify whether to start on the first or last day of your period. This method was developed by a Dutch physician in 1744.Īccording to the original rule, you must count 7 days from your last menstrual period and add 9 months to calculate your due date. In the past, most doctors and midwives used Naegele’s rule to calculate a woman’s expected date of delivery. By then, it’s usually safer to induce labour than to wait. Doctors will usually offer to induce labour between weeks 41 and 42. In theory, however, no pregnancy will exceed 42 weeks and 6 days (300 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). For a second pregnancy, delivery usually occurs around 40 weeks and 3 days.

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

How reliable is the estimated delivery date?Īfter 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.

How do I calculate my due date? Determining a specific date is more difficult than you might think.

3 days to due date