Concerning Ketubot page 39A: one of the 3 situations which justifies contraceptive use of a cloth:
Medicine today does not support the idea that there is such a thing as a second pregnancy, much less one that will cause the first to be squashed flat like a flounder.
How can one deal with such places where the Talmud presents as true facts or theories which do not agree with what we know based on medical science?
Joel
These discrepencies generally fall into one of three categories.
1. In some cases we may say Nishtanu ha'Tiv'im, that the physical nature of people has changed. (See Tosfos Moed Katan 11a DH Kavra)
2. In other cases, such as some of the remedies mentioned in the Gemara, the effect was not purely medical. Sometimes there may be a hidden reason for using or abstaining from a certain substance, and Chazal just gave the common knowledge reason.
3. In other cases, the medical "fact" upon which the modern approach is based is flawed, as we find changing medical 'truths' over time. An example of this is the symptom based treatment of many illnesses rather than the general health approach, and the value of individual nutrients.
As for the specific case you mentioned, it might belong to this last category. A friend showed me a medical report from the Dec. 19, 1997 Jerusalem Post ("Postscript" section) in which a birth of twins is reported, one of which was conceived a month before the other. According to the article, this was the sixth such pregnancy reported in recent medical history.
Alternatively, this Gemara could possibly relate to the first category, and such occcurences may once have been much more common.
By the way, the idea of a second pregnancy, although it appears in the Rishonim's explanation of "Sandal", is not brought anywhere in the Gemara. There are, in fact, alternate Perushim in the Rishonim for why the fetus will be 'flounderized'.
Dov Zupnik
What motivated Chazal to rule that a pregnant woman should use a contraceptive. Apparently, their reason was pikuach nefesh (of the first fetus), and as "Gan Hamelech" (paragraph 130) puts it, "most of women cannot be impregnated once more during a pregnancy... but in the matter of danger one should fear even a minor possibility, [i. e.] that the fetus will be squashed like a flounder."
However, as Rabbi Yitzchak Aizick Halevi Hertzog OBM explained in his responsa "Heichal Yitzchak" (Even HaEzer, part 2, paragraph 16), "this is not really a danger to life, for we have no life here, as a fetus is not considered a living soul," and he brought an argument from the Gemara in Sanhedrin 57b, which states that only Gentiles are punished for killing a fetus - but the Jews are not, as it is not yet considered a complete human being (see Rashi ibid., DH "Af al haubarin"). He also brought a proof from the Gemara in Erchin 7a which states that if a woman had started giving birth and died, one may bring a knife via Reshut HaRabim (thus violating the Sabbath) to cut her belly and pull the child out, but if she had not started giving birth, one may not violate the Sabbath to save the fetus, for in this case there is no pikuach nefesh. So, concludes Rabbi Hertzog, the rule that a pregnant woman should use a contraceptive is motivated by Chazal's will not to abandon the commandment of "be fruitful and multiply" (which would be done, were one of the two fetuses squashed).
[Why would the Mitzvah of "be fruitful and multiply" override the prohibition of using a contraceptive? Besides, the possibility of squashing the fetus is certainly a minor one, so we are not even certain to prevent the positive command from being fulfilled.]
The prohibition of using a contraceptive also stems from the commandment "be fruitful and multiply."
Dov Z.