Tosafos on Sanhedrin 59a has the following discussion:
ועל העוברים דעובד כוכבים חייב וישראל פטור אע"ג דפטור מ"מ לא שרי מיהו קשה דאמרינן בפרק בן סורר ומורה (לקמן סנהדרין דף עב:) יצא ראשו אין נוגעין בו דאין דוחין נפש מפני נפש אבל קודם שיצא ראשו החי' פושטת ידה וחתכתו לאברים ומוציאה כדי להציל את אמו וכה"ג בעובד כוכבים אסור כיון שהוזהרו על העוברים וי"ל דהא נמי בישראל מצוה כדי להציל ואפשר דאפילו בעובד כוכבים שרי
Essentially, the gemarra says there's nothing that is forbidden for a non-Jew and permitted for a Jew. Tosafos addresses the fact that a Jew is exempt if they perform an abortion, whereas a non-Jew is executed. Tosafos clarifies that a Jew is also forbidden, just exempt from capital punishment.
However, the gemarra says that as long as the head didn't come out, it's permissible to abort a baby to save the mother's life. Tosafos assumes this isn't permitted for a non-Jew. This seems to break the rule that nothing is forbidden for a non-Jew and permitted for a Jew. Tosafos explains that this is an exemption to the rule, for this is a case where it's a mitzvah for the Jew to save the mother. Something can be forbidden for a non-Jew, yet a mitzvah for a Jew, as we see by Shabbos (see the earlier comments of Tosafos, which I didn't copy).
Tosafos then ends off that perhaps it's permitted for a non-Jew to abort the fetus to save the mother.
What exactly is Tosafos' doubt? I'm looking for Achronim who speak this out.