Actually, we have several members on the board who have had successful pregnancies. For some, the UC gets better during the pregnancy, while others who have been in remission flare. Like everything with this DD, it's very individual.
The odds are low because this is not an "inherited" disease as such. A predisposition to it is believed to be genetic and sometimes inherited, but it also requires a "trigger" to activate the disease. No one is sure what all the triggers are at this point. Just because someone has the genetic predisposition doesn't mean they will ever actually develop UC.
As for medications, both Mesalamine (oral or rectal) and Prednisone have been used for decades and are accepted as not harmful to mother or baby. Immunosuppressants and biologicals have some risks which have to be weighed agains the risks of a severe flare while pregnant. Generally only a severe flare which produces dehydration, anemia and/or malnutrition would threaten the continuation of the pregnancy.
Only you can decide what level of risk is acceptable to you.