I've read somewhere that Catherine of Aragon would often engage in religious fasting while pregnant, which may have contributed to her miscarriages and sickly children and consequently the whole "I need a son" situation on her husband Henry VIII's part.
So, what if the Catholic Church had forbidden religious fasting by pregnant women, citing the risk to their unborn children?
(This assumes a certain degree of knowledge on their part that might not have existed at the time.)
Not sure what possible effects it would have beyond (possibly) giving Henry VIII a male heir and thus preventing the break with the Catholic Church from occurring as it did.