When we were dating, the first thing my now wife did was get me on a regimin of sunscreen and tretinoin. I now have significantly fewer wrinkles than my younger siblings (2-3 years).
This is a common misconception. Topical tretinoin has been shown to have no effect on the fetus. Oral tretinoins (like isotretinoin) have been linked to congenital defects. This is due to the differing rates of absorption and the resulting ability for the retinoid to cross the placenta at higher concentrations.
With that said, folks who are pregnant are discouraged from using any retinoids out of an abundance of caution.