RE Riding and pregnancy:
From a medical point of view, (Sorry the ER RN comes out at the darnest times) there comes a point at which the baby can put enough pressure on the cervix to cause the membranes to rupture in a bumpy situation. In many cases when there are no complications, women can ride up to 4-5 months safely if they are not putting to many G's into things. There is always risk at any stage.
Third trimester should be out of the question though. Each woman needs to make that decision with her OB and the Father of the child (if he's part of the equation). Most women get a feel for when they need to stop.
Depending on the type of bike she rides, the size of the baby will put pressure on internal organs, like the bladder, and add a bump and OOPS !! Something feels warm and wet. Is it raining with the sun out? Can't resist the humor. This did happen to a friend of mine, that was her last ride until after she stopped breastfeeding. She was just about 4.5 months.
Remember, each woman and fetus will be different and the same woman may be different with each pregnancy.
Just food for thought.