Well, the English wiki isn't very helpful, since it states she's a druid, a healer and a sorceress at the same time. The German version is more specific:
"Visenna is a sorceress, who specialized on the field of healing through magic. She is of juvenile beauty [a typical trait of sorceresses] […] Most sorceresses can't bear children. Only a few are an exception; Visenna didn't know this, though, when she conceived Geralt. […] Geralt is slightly embittered because Visenna gave him away back then – while she speaks of destiny having met again."
For those who know German:
Visenna
So the German entry doesn't make a difference between a druidess and a sorceress, or there truly are exceptions. I guess the issue also depends on how consequent Sapkowski is with his own "rules", with the inner logic of the world he created.
Still there's no definite motivation given why she brought Geralt to Kaer Morhen.
@Mow2345
You are right; I forgot about that passage. Yennefer also describes Geralt as self-pitying at times.
On the other hand, I found beneath the entry a quote from
Sword of Destiny:
"Calanthe: Do you hate that woman, Geralt?
Geralt: My mother? No, Calanthe. I can imagine she had to choose. . . . Maybe she had no choice? No, she could choose, […]. The choice. The choice that must be respected, because it is the holy and unassailable right of every woman. […] But I think, meeting her, the face she would now make. . . . That would give me a kind of odd pleasure."
That passage is up to interpretations. At least it looks like he intended to forgive her or even forgave her later on.