I don't feel it's the primary cause, but definitely plays a big part. I had just wrote an article explaining how if each couple has 2 children, then they grow up and have 2 children each, and that cycle continues, it exponentially grows out of control. Medication keeps people here longer than natural selection would allow for sure though.
I think medication for children would be the only part that really influences population. Once they are old enough and have children of their own, they start a whole new branch of the "family tree" instead of dying before ever having kids which would've prevented that entire future branch of the tree from ever starting.
So I don't think medical advances alone have caused it, but it does play a part in it.