Be young. Be naive. Be stressed out. Have your OB say, "Well, it's either this or a dead baby..." Have your family, your partner, your friends, all standing around waiting for you to "be a good girl and do what the doctor says..."
Then tell me it's simple to say no. Because it bloody well isn't.
YOU may be strong enough to still say, "Nope, no thanks!" in this situation, but please don't assume that everyone has that kind of fight in them and can stand up for themselves in the face of numerous issues which may even see the strongest of women buckle and cave into pressure and do something she doesn't want to do.