Every one gets these while pregnant. It forms on the ovary your egg is released from and it supplies the fetus nutrients until your placenta takes over. Id be more worried if you didn't get one. :-)
In fact, I had a small one for first few weeks of my pregnancy. Never caused an issue. That's also how they're able to tell which ovary the egg came from!
I've never heard of it harming the baby. Shouldn't interfere at all. They are very common, and usually go away on their own after a little while. The only time I think it might be of concern is if it is very very large. If the doctor didn't seem concerned, you're probably fine.