Q&A: How are the weeks of pregnancy determined?

Yes, this is totally confusing isn't it? When trying to understand gestational age standards, the best place to start is back in the days before ultrasound and ovulation detection, when the only thing we had to go on to determine due date was the last menstrual period. Ovulation occurs about day 14 of an average 28-day cycle. So conception on average occurs on day 14, or what we might call 2 weeks of gestation. It can be confusing to think that you get two weeks of "credit" before ovulation even occurred, but this is based on millennia of not being able to identify the day of ovulation, and agreed-upon standards of how to count gestational age using the last menstrual period.

The other confusion is that weeks are only counted after they have been completed, just like we report age after birth. A child has completed four years of life on her fourth birthday, and you have completed 40 weeks of pregnancy the day you turn 40 weeks. What is confusing is when we add "th" to the end of the weeks. When a child is 50 months old, she is "in her fifth year" but she is 4 years old (and two months). When you are 40 weeks 3 days you are in your 41st week of pregnancy, but you are not 41 weeks yet. Confusing, eh?

Because of so much confusion, the American College of Obstetrics and Gynecology is changing over to using only completed weeks as their standard. So when they say that elective induction of labor should never occur before 39 weeks, they mean 39 completed weeks, and not "in the 39th week." The new terminology is to say 39 weeks, 0 days, so it isn't so confusing.

To apply this to the example you gave: on average, conception occurs at 2 weeks 0 days. The period would be expected at 4 weeks 0 days. And your due date will be at 40 weeks 0 days. I hope this helps!