Are you positive?

I was vaguely aware that sometimes a baby's blood could somehow be incompatible with the mother's and cause problems, but I didn't really know what that was all about. So when it came up the other day, I decided to do some reading. Here's what I found out.

Most people's blood has something known as the Rh (Rhesus) factor, which is basically a protein that rides around on your red blood cells. If you have it, you're Rh positive; if you don't, you're Rh negative.

Being Rh negative isn't a big deal. Your body doesn't even know what it's missing. The only problem (that I know of) comes if you happen to be an Rh negative woman who is pregnant with an Rh positive baby and if your blood happens to intermingle. In that case, your white blood cells - who tend to be a little trigger happy and very xenophobic - go into attack mode and could seriously hurt your baby.

Anyway, if you are at risk for Rh incompatibility (more on that below), usually all you have to do is get an Rh immune globulin shot when you're 28 weeks pregnant and then again after you give birth. That pacifies your white blood cells and keeps everyone healthy. There are a few other instances (like if you have a miscarriage) where you might need that shot, but your doctor will be able to discuss that with you.

There's no way to tell for sure if a baby is positive or negative until birth. However, since it's a genetic trait, it's pretty easy to guess if there's an incompatibility possibility.

• Rh positive dad + Rh positive mom = always Rh postive baby (compatible, no worries)
• Rh negative dad + Rh negative mom = always Rh negative baby (compatible, no worries)
• Rh negative dad + Rh positive mom = maybe Rh negative baby (rarely a problem)
• Rh positive dad + Rh negative mom = maybe Rh positive baby (incompatible! Get your shots.)
• If the dad's Rh is unknown and you're Rh negative, assume you're at risk.

More info:

• the easy-to-understand article
• the medically precise article

PS The first photo is of blood cells, but don't you think it'd be an awesome pattern for a funky top?