Renal agenesis after first trimester exposure to chlorambucil.

One of a set of twins aborted at 20 weeks' gestation had unilateral renal agenesis after exposure to chlorambucil and prednisone since conception. There are no firm clinical data suggesting that second or third trimester exposure to any of the immunosuppressive drugs is toxic to the fetus, and no absolute cause-and-effect relationship is established in this case.