I had a baby 8 months ago. I measure a small now, but I'm 6 weeks pregnant. Should I go with a small or medium?

Go with the small. You're baby's older and sitting more on your hip and facing out, so you'll want to make sure your sling is fitted to you. A small sling will put your baby at a good spot on your hip, where a medium would be a little lower.  

You also won't have much room in the next few months to sling your kid in the front, so stick with a size that will be great to wear on your hip. (To check out the different ways to carry your baby just check out our "Wearing Instructions" link on the left sidebar.)