I always believe that it's better to be on the safe side. Do Kegel exercises religiously. The pelvic floor supports the bladder, uterus, and intestines. The added weight of the uterus during pregnancy can stretch out that floor, causing either the intestines or bladder to drop down. This is one of the reasons that so many elderly women suffer from incontinence.
Prevention is the best medicine.
Kegels involve contracting and releasing the PF muscles, similarly to stopping the flow of urination. Tighten and relax the muscle quickly several times a day. Deciding which activities are best for you is highly individual. During the later trimesters, running and cycling tend to become uncomfortable. Most of the women that I trained switched from these sports to swimming, hiking, water aerobics, and cross-country skiing.