One cannot do evil that good may result. One can do good even if the double effect of potential evil is foreseen (but this is pretty much a non-problem, as by the time such a case would incur the life of the mother being at risk, the child would be viable outside the womb and/or an attempt could be made to save the child.)

https://www.catholic.com/magazine/pr...-double-effect

Actual cases where a decision must be made between the mother’s life and the baby’s are rare, but they do occur, and there is always a moral response. Morally mature, ethical doctors are equipped to handle these difficult situations in the rare instances that they arise.

Legislation to regulate these rare occurrences has opened the door to abortion on demand. Statistics bear this out. The National Right to Life Committee reports that 93 percent of abortions are performed for "social reasons," while the mother’s health is cited in only 3 percent of the cases. (In another 3 percent the baby’s health is cited, and 1 percent cites rape or incest.) "Saving the life of the mother" never came up in the report (http://www.nrlc.org/abortion/facts/r...abortions.html).

The principle is simple: The direct killing of an innocent life is a grave evil and is never allowed, but when the mother’s life is in danger, medical ethics have always recognized the principle of double effect.