Pregnancy and lactation are the only body system functions I can think of which are potential in the adult. Everything else is actual: the heart pumps blood, the stomach digests, the intestines absorb, the kidneys filter, the muscles move, endocrines regulate, the nerves transmit...
And it is this potential function of pregnancy which is somehow set apart in the Church. You can remove an organ if it becomes dangerous or potentially dangerous to you, or even very inconvenient. A gangrenous limb - off. A cancerous lung - out. Breasts with a high risk of cancer - you may remove. A gall bladder that keeps making gallstones, or a uterus with bad fibroids - you may remove. A cancerous prostate whose removal will cause impotence - you may remove. Ovaries with a high risk of cancer - you may remove. Even a cancerous pregnant uterus you may remove. An ectopic pregnancy you may remove - if you treat it as a diseased fallopian tube and not as a pregnancy.
But if it is the state of pregnancy itself that is dangerous - if pregnancy would overtax a weak heart or kidneys - if it would with a very high likelihood cause hyperemesis gravidarum or severe early preeclampsia - tough, suck it up, offer it up. You may not remove the potential of pregnancy for the purpose of removing its potential danger.
Mrs VS
Or if the pregnancy, even without a threat to the mother's life, cannot end well for the child... as in the case of the mother with the incompetent cervix that Scotswife mentionned, or when one parent has a chromosomal translocation...tough.
I think the Church at least used to permit contraceptive pills to regulate women's irregular and unpredictable cycles - with the idea that a couple was entitled to predictability but should abstain at the times they would have if they were using NFP.... not sure that is still permitted.
Sometimes, the reasoning of the Church, as when it teaches that you may remove a tube with an ectopic pregnancy, but you may not remove the pregnancy from the tube, has me beating my head against the wall. I wonder, AFTER you have removed the tube, then can you remove the pregnancy from the tube, for the sake of science and advancing medical knowledge and seeing the baby and burying it?
It's late...
Mrs VS
"But if it is the state of pregnancy itself that is dangerous - if pregnancy would overtax a weak heart or kidneys - if it would with a very high likelihood cause hyperemesis gravidarum or severe early preeclampsia - tough, suck it up, offer it up. You may not remove the potential of pregnancy for the purpose of removing its potential danger."
I hadn't seen put that way before - but you are right.
"suck it up" does appear to be the message.
And if I can go back to my stomach stapling question....people get their stomachs stapled even when the stomach is functioning pefectly normally - no disease - no abnormality.