Since a Ben Noach is Chayav Missah for killing an Ubbar (fetus) and a Yid is not (its Assur but no Chiyuv Missah), a Goy should not do it (this is obviously when there is a Jewish Doctor of equal ability available to perform the surgery).
You are incorrect about it being muttar only in life threatening situations (ain kaan makom le’harich)
I will try and find you the exact Mekoros later (buried somewhere in my notes!)
This was told to me by Reb Zev Leff (however its pashut!-i.e. very straightforward mekoros in Gemara)