I am sending you the Kollel's response to a similar question:

(a) The sources we are dealing with have to do with two Talmudic concepts:

1. A "Ben Shemonah" is considered to be "dead flesh," or "like a stone" in the words of Chazal (Yevamos end of 80a).

Nothing in this statement shows that a fetus born before eight months is more viable than an eight month fetus. The Beraisa is simply saying that a fetus born before the ninth month is not viable since it has not yet matured properly in the womb (see Rambam, Hilchos Milah 1:13, Tum'as Mes 1:14). This is evident from the Gemara (ibid.) which specifies that an eight month fetus can be recognized by its deformed fingernails and hair.

The Beraisa lists the above deformations when discussing the opinion of Rebbi. But even the dissenting Rabanan maintain that the reason an eight month fetus is not viable is because it has not fully matured. They argue, however, that maturity is not visible on the fetus externally, and therefore even a fully formed fetus born before the ninth month may safely be considered not viable. The proof of this is that they, too, agree that there are rare cases in which such a fetus can survive and is considered to be fully alive. These exceptions to the rule are applied when the child lives either 30 days or 20 years (see Tosfos ibid. 80a DH v'Ha).

If so, certainly a fetus that was born earlier than eight months can be given the same non-living status if it was not fully developed; the Gemara means that even an eight month baby is not considered to be alive under such circumstances. (A nine-month baby may live even if it is not fully matured, due to its stronger constitution. Alternatively, an immature nine-month baby is also included in the category, and termed an "eight month baby" for this purpose)

In short, when Chazal speak of an "eight month baby," they are referring to a baby that has not fully matured in the womb and was born before the ninth month.

2. The second source under discussion is the Gemara's statement in numerous places (Rosh Hashanah 11a, Nidah 27a, 38b, Yevamos 42a) that a nine-month pregnancy is a full nine months - but a seven month pregnancy can end safely before the end of the seventh month.

This statement seems to be leading to the counterintuitive conclusion that there is a "twilight zone" between the end of the seventh month and the beginning, or even end, of the ninth, during which a baby cannot be born living. However, both before and after that period it can indeed be born alive.

Nevertheless, a careful examination will show that this is not necessarily the intent of this statement. Rather, it was cited to show that a child can be born healthy and whole not only at the end of the seventh month, but even at the beginning of that month (see the Gemara in Rosh Hashanah and Yevamos ibid.). Certainly, a child born after the beginning of the seventh month can be healthy and whole.

This is evident from the Gemara in Yevamos 80b which emphasizes the difference between an "eight month baby" and a "seven month baby that overstayed its time in the womb". That is, an eight month baby is one that has not matured. A seven month baby is one that has matured much more rapidly than the usual fetus, and is fully formed by the beginning of the seventh month. It is thus fair to conclude that the term "seven month baby" includes any fetus that has been fully formed - whether it is born in the seventh month, or in the eight or beginning of the ninth. If it is fully formed - it is in the category of seven month babies and what Chazal refer to as "an overstayed seven month baby."

In short, when Chazal speak of a "seven month baby," they are referring to a baby that has fully matured in the womb and was born before the ninth month.

(c) What about the Gemara in Nidah 27a and 38b that seems to imply that there is indeed a question as to whether a nine month baby can be born at the beginning of the ninth month (Yoledes l'Mekuta'in) or not?

After careful examination of the Gemara in Nidah 38b, it seems to me that the argument there is based on the following clear medical fact: Most babies (let us say, 80%) are indeed born at full term. The question there is whether such statistics should be taken into account (Halachically), or whether we can assume that babies are born when they are born, and not adjust our lives to expect the baby to be born specifically after full term. (The Gemara there is discussing the practice of Chasidim to procreate only on certain days in order to avoid having a baby born on Shabbos.)

The same may be said of the Gemara in Nida 27a. The Gemara simply considers it odd (according to the opinion that we do take the likelihood of full term into consideration) that we are told to expect babies to be born before full term. In either case, Tosfos there already points out that we see in Yevamos 80b that babies can indeed be born, healthy and full, even in the eighth or beginning of the ninth if they matured rapidly - as we pointed out above.

(Tosfos concludes that the Gemara in Yevamos is arguing with the Gemara in Nidah 27a; however this does not seem to be the consensus of the Rishonim. The Gemara in Nidah 27a which does not raise the possibility of an early term pregnancy may simply be offering another solution to its question.)

In short, when Chazal say that a nine month pregnancy ends at full term - they mean that there is so great a chance that it will end at full term, that we need not take short pregnancies into Halachic consideration.

(d) With all of this in mind, it is easy to see that there need not be any contradiction between intuitive and scientifically proven facts of pregnancy, and the words of Chazal. No need to invoke changes of nature etc. (at least according to most Rishonim; the Tosfos cited above may argue).

When we find the Rambam (Hilchos Milah 1:14, Perush ha'Mishnah Shabbos 19:5) discussing "a baby that is born, and it is not decisive whether it is an eight month baby or a seven month baby" - it means that it is not clear to us whether the baby is mature and therefore viable, or not fully mature at birth and therefore not viable. This is indeed clear from the Gemara upon which the Rambam's words are based (see Shabbos bottom of 135b). The number of days that the baby stayed in the womb is not under discussion.

(e) It is important to point out that when the Gemara says a fetus is "not viable" - it is referring to viability in a period during which the medical facilities did not use incubators and modern medicine. Nowadays, even a fetus born at a very early period of pregnancy can survive through the use of such modern facilities.

Bearing this in mind, there should be no question that one may be Mechalel Shabbos in order to save the life of such a fetus. If it has a good chance of living through the use of modern medicine, it is considered "alive" for all intents and purposes - as Hagaon Rav Shlomo Zalman Auerbach is quoted as saying (Shemiras Shabbos k'Hilchaso, I:35:24).

I hope this clears the air a bit on this subject.

Best regards,

Mordecai Kornfeld

Kollel Iyun Hadaf

What I have written deals with the resolving the logical paradox's inherent in considering an eight-month baby less viable than a seven-month baby. Even if nature changes, logic should not change.

As for Hipporates and the medical treatises to which you refer- I haven't seen the sources. For all I know, perhaps Hippocrates, too, used the words "eight-month fetus" in the sense that I outlined above. Can you check whether that is possible?

Best Wishes,

Mordecai Kornfeld

Very interesting - from what you suggest, it could even be a true statement... (after all, the nations would only have made such an illogical statement if they had evidence from experience). Here is what I found on the Internet at https://archive.org/stream/56811050R.nlm.nih.gov/56811050R_djvu.txt

ON THE SEVEN-MONTH BIRTH

- De Septimestri Partu, Fcesius, Treat, v. p. 255.

- De Septimestri Partu, Haller, ii., p. 90.

- De la Grossesse de Sept Mois, Gardeil, ii., 443.

Haller appears to think that in the time of Galen, the two books, " De Septimestri et Octimestri Partu," were regarded as one; in which he is supported by the authority of Fcesius. This production he contends, has given rise to the long prevalent opinion, that the foetus is stronger and more capable of living when born at the seventh than at the eighth month. If not then brought forth, it languishes for forty days, and is born after the ninth month. If, however, it is born during that interval, it is weak and cannot survive."

In short, it may definitely be so, and it may also be that they are referring to what I suggested.

Thanks, Mordecai