Actually, I'm going to disagree with you here. Contraception was a woman's worry--condoms were primarily used in the earlier days to prevent venereal disease. Women used herbs to bring on their periods or to abort unwanted fetuses (which was not usually considered a sin before "quickening"), they used sponges (frequently soaked in vinegar) or various types of pessaries to block the opening to the cervix: honey, wax, oil, crocodile dung(!). As today, oral and anal sex were also used to avoid pregnancy.
If your female character is fertile and sexually active, I think some passing mention of contraception would be enough. And hey, since it's fantasy, you can invent an herb or spell that works every time.
Well, I will disagree back at you.
I don't know any way on earth a woman can be responsible for coitus interruptus or anal sex and generally requires cooperation in replacing vaginal sex with oral. I don't think it could be said to be something a woman could be responsible for although of course she could either demand or encourage it.
Incidentally, my reading indicates that it was learned quite early that condoms prevented pregnancy in addition to being used for disease prevention.
Coitus interruptus was the means probably most widely used in a historical context judging by mentions in widely diverse writings.
Women who gave herbs to bring on an abortion were highly likely to end up burned at the stake as a witch in the real world--which wouldn't be a problem in a fantasy but bad worldbuilding would be. Honey and that nonsense didn't work although pessaries made of dung surprisingly enough may have had some effect since they were alkaline, but hardly something I'm going to put in a novel nor is my world primitive enough for that to be believable.
Condoms and withdrawal did work at least a good part of the time.
No, I would never just put in any old spell that will work every time. The magic in my novel is elemental based not alchemical or herbal magic. It would be totally out of place and be inconsistent which is a serious fault in a fantasy. As Shweta pointed out, the method used should be consistent with the world and the magic type, not just randomly tacked on.
I have decided to go with condoms, a passing reference should do. I wish I had Veinglory's way with sex scenes though.
Edit: I agree, Veinglory. I thought the whole "berry" thing in Auel's books was quite implausible.