Yevamos 78a - Milah After Tevillah

The gemara explains that even if an unborn child is not part of the mother, and the conversion of the mother does not work for the child, the tevillah of the mother works as if the child himself had tevillah and therefore undergoes his own conversion. The Nimukei Yosef (bottom 16a) cites Ramban who proves from here that tevillah can be done (bidieved) before milah, and he would not need a 2nd tevillah. However, the Re'ah argues and says:

דכשהוא במעי אמו אינו בן מילה, והרי הוא כנקבה דסגיא לה בטבילה, וכשנולד ומלין אותו אינו אלא כמו שמלין ישראל שהוא ערל...

Milah is only required when it is physically possible to do it, but so long as the child is in the mother's womb he is not an o'rel and can be converted with tevillah alone.

Tosafos (47b on top) can either be explained to be saying like the Re'ah, or can be explained to mean that when milah cannot be done at the time of tevilla, we would allow the tevillah to be done before the milah, but both are required for the geirus. The nafka minah is whether the milah has to be done l'sheim geirus.