I was taking prozac (fluoxetine) when i conceived my son 12 years ago (up to apx 4 weeks into preg). he has behaviour problems and wondered if this could be why.?

Probably not: Only two studies i know of. Both from the same group. One was with Prozac specifically. The authors concluded that their findings support the hypothesis that fluoxetine and tricyclic antidepressants are not behavioral teratogens and do not have a significant effect on cognitive development, language or behavior.

Doubt it: Prozac (fluoxetine) taken during most of pregnancy has been linked to transient withdrawal symptoms bu little else.That said, exposure timing has a lot to do with outcome. The fetal brain has not started forming the areas responsible for behavior by 4 wks so a causal effect is hard to accept. Behavior issues are derived thru a complex interchange of life experiences & per-wired responses. Hope this helps.