By not donating blood, you are allowing some people to die or to suffer more than they otherwise would. By not donating organs or bone marrow you do the same. Should you thus be forced to undergo this donation? I would argue that it would be wrong to force you to do this, even though it would save lives, because you have the right to bodily integrity. I also think that women have the right to bodily integrity, that women should not be forced to do things with their body that they do not want to do. If you happened to find yourself in a hospital for an operation, and someone comes in to the hospital that desperately needs a kidney, then even if you are in surgery already, doctors should not take your kidney without your consent even though it would save a life. But you wish to make sure that the rights of the woman are always below that of the fetus.