When I first noticed a mark on his leg at 2 weeks old, it was just a very small crescent. I thought maybe he had gotten his leg pinched in the carseat strap or someone had accidently scratched him or something. A few weeks later, I noticed that it had grown instead of getting better so I mentioned it at his 1 month appointment. The doctor said it is a birthmark called a capillary hemangioma, also referred to as a strawberry birthmark. This type of birthmark forms about 1-2 weeks after birth and starts growing at a month. It can grow for 6 months up to a year and then should begin to shrink back on its own, usually resolving itself by age 10 or so. Hailey calls it "baby brother's strawberry."
Here it is at a month, after it had grown a bit from its original size:
Here it is at 6 months:
It pretty much seems to have stopped growing at this point, so now we'll just wait for it to start shrinking back. I don't really notice it anymore, and it is such a part of him that I think I will miss it when it is gone.