Our ship had mechanical problems in October 2006 so we missed a port. We were reimbursed the cost of our missed excursion and whatever other related charge (I guess port tax). Petitions on board don't do anything. When you get home, remember your etiquette and write a letter and see what happens. Remember for every bad thing you say, you should mention something good too. You may have a surprise waiting for you on your next cruise.
By the way, no matter what the medical emergency is or who is having the medical emergency, employee or otherwise, the medical emergency will take priority. What if it was your life.
As for pregnant cruisers, our group found out the hard way, with an unexpected pregnancy in the group, that you are not allowed to cruise unless you are beyond your 24th week.