After a woman discovers she’s pregnant, her next question is likely, “When am I due?” Your obstetrician should perform a vaginal ultrasound at your first prenatal visit to determine exactly how far along you are. A date based on fetal measurements is generally much more accurate than other means, such as using the date of your last menstrual period (LMP). Your LMP does, however, allow you to get a rough estimate at home, rather than having to wait for your doctor or midwife. Here’s a look at how to estimate your due date, how accurate this method is, as well as the pitfalls of calculation.
As long as you know the start date of your last period, this is a very easy estimation. To calculate your due date, figure out the first day of your last menstrual cycle. Then, count forward 280 days from the first day of that particular period. This includes the average gestation period (266 days) and an additional two weeks, the amount of time it takes for a woman to ovulate and menstruate. If you’re actively attempting to conceive, keeping track of your cycle can make this easier.
Using your LMP calculates the “gestational age” of your baby, as opposed to fetal age. Fetal age is the actual age in weeks or months of the fetus. Gestational age includes the time during which an egg is released and ovulation occurs -- the prime time for conception and fertilization. This two week process creates a fourteen day discrepancy between fetal age and gestational age, which can be confusing.
This method is best used if you have a regular menstrual cycle that runs in 28-day cycles, with ovulation normally occurring right in the middle of the cycle on day fourteen. Not all women have regular cycles, or they ovulate more or less than two weeks after the first day of their period. This can make it hard to tell on exactly which date fertilization took place, leaving you with a relatively inaccurate due date.
Although it should be pretty close, your real due date could be weeks before or after the LMP estimated due date. Women with shorter menstrual cycles may calculate their due date to be further off than it really is, while women with longer menstrual cycles may calculate sooner than actuality.
Spotting and hormonal changes may also influence how accurate your calculations are. Spotting is light bleeding between periods, which some women experience as a result of ovulation. The LMP method can also be difficult to use for women who have been on birth control pills or are breastfeeding. Both of these situations alter hormone levels, which can compromise the cycle the body generally follows. Even women with perfect 28-day cycles don’t always ovulate exactly in the middle, making the calculation inaccurate despite regularity. Determining your due date via ultrasound is generally much more accurate than using your LMP, which doesn’t depend on menstrual regularity to make accurate predictions.