Maybe, maybe not. Depends on your campaign.
I know that in the Forgotten Realms, for one example, there are fairly-easy-to-obtain herbs that can suppress fertility, for both males and females (detailed in the FRCS, which I don't have in front of me at the moment).
If we assume that many of the things that modern medicine can accomplish (i.e., healing injuries, curing diseases) are provided in the D&D world by divine magic and / or alchemy, it's fair to assume that birth control, too, can be provided in a similar fashion.