Why does Raban Shimon ben Gamliel consider a less than 30 day infant a "Safek Nefel" - to the extent that the Gemara considered prohibiting giving babies Milah on Shabbos. Why don't we follow the Rov? Certainly, Raban SBG must admit that the mortality rate of infants is less than 50% - as Rashi indeed remarks in DH Nafal Min ha'Gag, according to the Rabanan who argue with Raban SBG?
Q. Reese, Atlanta, Georgia
The exact case of Raban Shimon Ben Gamliel is argued by the Rishonim. Some, like Rashi (Yevamos 36b) and the Ritva on our Gemara, understand that Raban Shimon Ben Gamliel would only say his Halachah if we are aware that the baby could be considered a baby which was only carried for eight months (which everyone holds generally does not survive) or one that was carried for nine months. If the baby could not possibly be an eight month baby, Rebbi Shimon would agree that it is considered "Chai" from day one. Being that we are unsure how long the mother carried the baby, we cannot say that we apply a Rov (as this is a factor which weakens the Rov). The Rabanan say that although it is possible that the baby is an eight month baby, we focus on the general Rov that most women have healthy babies, and ignore the eight month baby possibility.
Even according to other Rishonim (Tosfos), who say that Raban Shimon Ben Gamliel is talking about all children, one can answer this question as does the Chasam Sofer, who explicitly asks your question. He quotes in the name of Tosfos that because it is a "Miut ha'Matzuy" -- "common minority" that some babies are Nefalim, Raban Shimon Ben Gamliel held that mid'Rabanan we should not give babies the status of a Chai until after thirty days.
Take Care,
Yaakov Montrose