It’s one of the most common questions pregnant women are asked: “How far along are you?”
Most people think about pregnancies as 9 months in length. But obstetricians think in terms of weeks, not months. And the funny thing is, we start counting before the woman has even conceived.
The countdown starts with the first day of the last menstrual period, because it’s more common for women to monitor the timing of their period than when they are ovulating. Historically, that was a time a woman might be able to recall. Forty weeks (or 280 days) after that date, most women would deliver their child.
Ovulation happens about two weeks after the countdown begins. As a result, expectant mothers aren’t actually pregnant for the first two weeks of the 40-week span.
This, of course, can lead to confusion. If we tell a patient she is 12 weeks pregnant, she might think we are telling her she conceived 12 weeks earlier. This might not match up with her recollection of events.
Some patients also try to convert the timing from weeks into months. For example, a patient who has her 20-week appointment may think she is 4 ½ months along, because 20 weeks is half-way to 40 weeks, and 4 ½ is half-way to nine months. But there isn’t a direct correlation, and this adds to the confusion.
My advice: It’s a lot easier to just think in terms of weeks. Measuring a pregnancy as 40 weeks is more accurate than measuring it as nine months. If you want to know what day of the week your due date will fall on, just ask your obstetrician which day you “rollover” to the next week. It will be that day, unless it’s a leap year.
A full-term pregnancy lasts between 40 and 42 weeks. There are pregnancy calendars on the Internet that will calculate your due date, but you can do it yourself with some quick calculations. Here’s how:
- Determine the first day of your last period (August 1, for example)
- Add one week (August 8)
- Move back by three months (May 8)
- May 8 is your due date
Assigning a due date by this calculation assumes the mother had regular 28-day cycles, that she ovulated on Day 14 of the cycle, and that she got pregnant right away. The truth is, some women have irregular periods or longer cycles, and the time to fertilization can vary slightly. Also, a small amount of vaginal bleeding can occur during implantation of the very early pregnancy, and it can be confused with a normal period.
Fortunately, we live in the era of modern medicine. Ultrasound and sensitive pregnancy tests can detect pregnancy very early and help assign an accurate due date.