40 Weeks, not 9 Months
So, it’s not 9 months of pregnancy, but rather 40 weeks of pregnancy. Because the pregnancy begins on the first day of the last menstrual cycle, the total comes to 40 weeks. This varies from conventional wisdom which tells us that a woman is pregnant for 9 months. This is an important distinction when dealing with healthcare professionals.
A Normal Menstrual Cycle
Most due date calculators assume a “normal,” 28 day cycle when calculating your EDD. However, normal varies and typical adult women have cycles that range anywhere from 21 to 35 days. This normal variation is not accounted for in due date calculators.
Length of Cycle Affects the Due Date
The length of the menstrual cycle affects the due date. Some common variances include infrequent periods and too frequent periods. All of these variations can affect the EDD. For example, women with an irregularly short cycle tend to deliver a few days earlier. Women with an irregularly long cycle tend to deliver a few days later. Due date calculators do not take these variances into account.
Due date calculators typically assume that conception occurs on day 14 of the cycle. For many women with a normal cycle this might be correct. However, because of the variations in cycles, the EDD can be subject to an error of more than 2 weeks. In fact, the most fertile period is a range of days, days 10 to 18 for many women. It is likely that ovulation occurs at some point during that range of days.