You’ve always known pregnancy to last nine months, so why do we list 10 months? Nine months is the common measurement for pregnancy, but pregnancy doesn’t divide itself evenly into months. In determining your due date and monitoring your care, your care provider works from a 40-week calendar (this is explained further in Chapter 4). If you consider that most months are four weeks long, take 40 divided by 4, and that equals 10 months. In actuality, though, some calendar months are five weeks. So, if you track your pregnancy by calendar months, it often comes out closer to nine months.
Really, though, your care provider is more interested in what week you’re in, because months are too imprecise. If you say you’re in your “third month,” that could mean you’re at week 9, 10, 11 or 12 of your pregnancy. To determine if your baby’s growth is on track, your provider needs to know the exact week, or close to it. Also, some testing has a narrow window (such as between weeks 11 and 14) during which it needs to be administered.