I´ve heard that taking Asacol (mesalamine) (for Ulcerative colitis) can affect the baby´s growth during pregnancy. Is that correct? And what about breastfeeding?

Below. The effect that Asacol (mesalamine) has on an unborn child has not been studied extensively. Asacol (mesalamine) should only be used during pregnancy if clearly needed. Asacol (mesalamine) does pass into breast milk, and could affect a nursing infant In short---we really do not know. Best not to take unless absolutely necessary. Best not to breast feed while taking.
Asacol (mesalamine) It is category B in pregnancy, meaning animal studies didn't show any problem but no available human studies, your doctor would weigh risks vs benefits, usually safe to use. During breast feeding, no available data to suggest its safety, again ask your doctor then, if you would use an alternative, wish you wellness and your baby.