1. It is precisely for this reason -- that the 9-month baby is more common -- that the Gemara did not use this example. In that case, the Safek would no longer have been an "equal Safek" ("Safek ha'Shakul"). Since a 9-month baby is more common than an 8-month baby, it would follow that in the case of a doubt whether the baby is an 8-month or 9-month baby, we would say that it is likely to be a 9-month baby. and therefore we would desecrate Shabbos in order to save it. It is only a Safek of a 7- or 8-month baby that can be considered as an Safek ha'Shakul for which we still could say that there is a reasonable chance that it is an 8-month baby, and consequently we may not desecrate Shabbos to save it.

2.

a. We may add another reason for why the Gemara does not mention a Safek that the baby might be a 9-month baby. This is based on the Magen Avraham (Shulchan Aruch OC 330:15). The question is asked: why do we do not desecrate Shabbos for a doubt of a 7-month or 8-month baby? There is a principle that we always desecrate Shabbos for the sake of saving a life, even when there is only a small chance of saving a life. In fact, the Shulchan Aruch there (330:5) rules that if the mother died in childbirth and the baby has not yet been born, we desecrate Shabbos and perform a caesarean operation in an attempt to save the child. Even though the baby was not yet born, we assume that it is still alive and we may desecrate Shabbos because of this possibility. Why, then, do we not desecrate Shabbos for the Safek 7-month or 8-month baby?

b. The Magen Avraham answers that there is a distinction between whether the baby was carried to full term or not. In the case where the mother died in childbirth, the baby was a 9-month baby. Therefore, the baby is considered a full-fledged person and we may desecrate Shabbos even for a small chance that we might save it. In contrast, in the case of a Safek 7- or 8-month baby, neither possibility was ever a full-fledged being, and it follows that in the case of a Safek we may not desecrate Shabbos to save it.

c. It follows that if the baby was a Safek 8- or 9-month baby, we may desecrate Shabbos because of the possibility that it is a 9-month baby and a full-fledged person, even if there is only a small possibility that it is a 9-month baby. Certainly one may desecrate Shabbos since, in reality, it is more likely that it is a 9-month baby (which is more common).

Apologies for the delay in answering this question.

Kol Tuv,

Dovid Bloom

1. No, science does not override Halachah. Rather, the Halachah itself says that sometimes "Nishtanu ha'Teva'im" -- elements in the natural world have changed. In other words, the physical reality today is not always identical to the physical reality that existed in the time of the Talmud.

2. An example of this is the Mishnah in Bechoros (19b) which states that if someone buys a cow which is under three years old and it is not known whether this cow has already given birth (and therefore one does not know if the first calf subsequently born is a Bechor and must be given to the Kohen), we assume that it has not given birth, because cows under three years old never give birth.

3. Tosfos in Avodah Zarah (24b, DH Parah) expresses surprise at this, because, he writes, in his times (approximately 13th century, or some 1000 years after the Mishnah was redacted) it was very common for 2-year-old cows gave birth. Tosfos concludes that the nature of cows in his time was different than the nature of cows in the time of the Mishnah, and in his time it was common that they would bear offspring a year younger.

4. This phenomenon is also reported in the 16th century by the Shulchan Aruch (EH 156:3). The Rema there writes that although, according to the Gemara, only a baby born at the end of the ninth month is viable, in his time a baby born even one day into the ninth month is viable. The Rema writes that we must conclude that the physical nature has changed, and that this observation applies to a number of other matters.

5. The Chazon Ish (d. 1953) writes concerning our times (Chazon Ish, YD 155:4):

"Some mistakenly think that a baby born before the ninth month is a Nefel and they despair of saving its life and are not eager to heal the baby. This is a mistake. If its hair and fingernails are developed... one is obligated to try and cure it, and one must desecrate Shabbos to try to cure the baby. There are now many case studies of babies born in the eighth month who survived... because Nishtanu ha'Teva'im, nature has changed."

Kol Tuv,

Dovid Bloom