I’m an irritable person, I admit it, and I’ve been known on occasion to get all het up about trivial things that don’t affect me in the slightest. The way that some women calculate the advancement of pregnancy is one of those things. Consider yourselves warned.

Okay, so a term pregnancy is supposed to be 40 weeks long. Roughly speaking, this translates to nine calendar months. This seems fairly straight-forward, I know, but an astoundingly large number of people are unable to count it in a way that makes sense. For instance, I have known many, many women who count four weeks as a month, saying, then, at twenty weeks, that they’re five months along. Sorry, no, that doesn’t work unless you’re counting lunar months, in which case you need to tack an extra month on there at the end and consider your pregnancy a ten-month affair.

Along similar lines, it’s not uncommon to see women post on the pregnancy message boards that they are, for example, 20w0d pregnant (that is, twenty weeks, zero days), and interpreting that to mean that they are “21 weeks along.” Again, no. At 20w0d, you are precisely twenty weeks along. Think of it this way: on your twentieth birthday–when you have reached the age of 20 years, 0 days–you don’t start telling everyone you’re 21 (much as you might wish that were the case). You have embarked upon your twenty-first year, true, but you won’t be 21 until you have completed that year. Make sense? (In the same way, you are not six months pregnant until you have completed six full months of pregnancy — though it may be confusing for some, having completed five months of pregnancy makes you five months pregnant, not six. Yes, you are in the sixth month, but that’s not quite the same thing, now is it?)