the issue i have with this question is the same issue i have with many morality questions. it assumes a dichotomy of moral or immoral (some do have a third neutral option but not most) the thing is, something can be more moral than something else. a decision can be less immoral than another one, but still be immoral. so with that in mind, obviously giving up your life to save someone elses is the MORE moral decision. deciding not to though, is in no way immoral.
Immoral is usually used to describe morally impermissible actions.
Moral is usually divided into the broad categories of:
Required: Aka refraining from this action is immoral
Supererogatory: Aka this act is morally praiseworthy
Neutral: Aka permissible actions that are neither required nor praiseworthy
So the 4 major categories are Impermissible, Required, Neutral and Supererogatory. Obviously there will be differences of magnitude between actions within a category. Some complex actions are made by the sum of smaller actions that fit in these categories but the combination tends to also fit as a unit in one of these categories.
Mother's options in this scenario:
1) Die and the ___ will be born.
2) Abort and she will live.
Common categorizations on this topic:
Supererogatory, Neutral
Neutral, Neutral
Required, Immoral