I would like to know how months of pregnancy are calculated. I am 16 weeks, so I consider myself to be four months and about to be five when I get to the 17th week. I've heard that pregnancy lasts for 10 months and not nine as always thought. And some books say the first trimester doesn't end until the 14th week. I am confused. When am I out of the danger zone of miscarriage?

--Necole

We doctors do not think of pregnancy in months, but rather in weeks. We use weeks because it's a precise measurement -- every week is seven days long, but months can vary (February, with 28 days, is the only true four- week month!). If every month were exactly four weeks long, then pregnancy would indeed last 10 months, but that's not how it works on the calendar.

The number of weeks is calculated from the first day of your last menstrual period (LMP), for the simple reason that most women know when their last period began. Now, obviously you aren't even pregnant yet on the first day of your LMP, but this gives us a good, consistent reference point and is the reference point used all over the world. By using this as the starting point, we calculate your due date to be 40 weeks from that day.

Each trimester lasts 13 weeks (well, 13 weeks and 2.33 days, if you really want to be exact). Most miscarriages occur in the first trimester. At 16 weeks, you are pretty safe.