How is due date calculated?
Lots of us assume that a pregnancy is exactly nine months long, but that’s not the case. To work out how to calculate pregnancy weeks, there’s a little more to it.
“The nine months of a pregnancy are actually 40 weeks,” Dr. Charlsie Celestine, Flo board member, obstetrician, and gynecologist (OB-GYN), explains. “The due date is 40 weeks from the first day of your last menstrual period. But some women can go beyond that to 41 weeks.”
In fact, the first thing you’ll likely notice when you let your health care provider know you are pregnant is that pregnancy is calculated in weeks rather than months. And your baby’s estimated due date falls on the 40th week, when you’ll actually be around 10 months pregnant.
That’s to account for the fact that pregnancy is measured according to gestational age, not fetal age. So that means you count pregnancy from your LMP, not the date you conceived, adding an extra two weeks even though you weren’t technically pregnant then. Also, this method recognizes that not all months have the same number of days, so you’ll likely still be pregnant at nine months.
You might also see figures like 13/5 or 13+5 in your doctor notes. Pregnancy is counted in complete weeks, so 13/5, 13+5, or a variation of this would mean you’re 13 weeks and 5 days pregnant. Learn more about how you count pregnancy weeks here.
Your health care provider will usually calculate your due date based on one or a combination of the following methods, so let’s find out more about how they work.