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There is a rule in the Gemara (Yevamot 34a) that אין אשה מתעברת בביאה ראשונה. The question is why the Talmud assumes that a virgin cannot conceive on her first act of sexual intercourse if the hymen is actually penetrable and modern-day observations have proven that a women can indeed conceive then?

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Please cite (and/or link to) your source. Thanks! –  Seth J Nov 15 '11 at 19:34
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@avi: the woman in that case isn't necessarily a virgin. The Gemara that the OP is talking about is referring to one. –  Alex Nov 15 '11 at 20:03
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Quote just says "isha"... –  avi Nov 15 '11 at 20:08
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@avi: but it also says מביאה ראשונה, which means her first act of intercourse in her lifetime - not the first one with this particular man. –  Alex Nov 15 '11 at 20:41
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@avi: the sugya is Yevamos 34a-b. In the course of the sugya the Gemara asks how Tamar could have gotten pregnant by ביאה ראשונה with Yehudah, and after giving an answer (מיעכה באצבע), it asks: "But weren't there Er and Onan [her first two husbands]?" - i.e., "so she was already no longer a virgin; why are you asking about ביאה ראשונה?" and answers that Er and Onan had been intimate with her only שלא כדרכה. The obvious implication is that ביאה ראשונה here means her first act of (natural) intercourse, otherwise the question about Er and Onan is a non-sequitur. –  Alex Nov 15 '11 at 21:04
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3 Answers 3

This Daf Yomi Digest writes that

The Noda B’Yehudah writes in his responsa נודע יהודה קמא אה"ע סימן כ"ב that nowadays it is possible for women to become pregnant the first time they have relations. Furthermore, even in the time of Chazal the principle did not indicate that it was impossible for a woman to become pregnant the first time she had relations, rather the principle was stated with regards to the majority. The majority of women would not become pregnant the first time they had relations, but there was always a minority of women who would become pregnant the first time they had relations. This approach was followed by Rav Moshe Sofer, the Chasam Sofer in his responsa אה"ע חלק א סימן ו.

Others - שו"ת תשורת ש"י מהדו"ת סימן ע"ב, שו"ת מהרש"ם חלק ד סימן ע - write that in the time of Chazal the principle was absolute and the nature of people has changed (נשתנו הטבעים), which now allows women to become pregnant the first time they have relations.

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Do they mention what anatomical features changed to allow this? –  Double AA Dec 29 '13 at 3:02
    
I'm wondering how the concepts of rov and chazakah play into this. I heard Dr. Nancy Sniderman on Today say that the odds of pregancy are 1:8. If that ratio was recognized in the past one could not say that pregancy never occurs on the first act, but that it is less likely than not and impossible to assume. –  Bruce James Dec 30 '13 at 20:20
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I wonder if it might just be a matter of timing. The egg lives for only a maximum of 24 hours or so after ovulation, and sperm also are fertile for only a couple of days. Now, a woman might get married, and have her first act of intercourse with her husband, anytime during the tahor part of her cycle (and, according to the original Torah law, that would be 3 out of every 4 weeks), so the chance of this occurring close to her ovulation would be relatively low. During the rest of her married life, on the other hand, there's a much higher chance that they'll have relations at (or close to) her fertile time.

So this could indeed be expressed as the statistical fact that conception after first intercourse is much less likely (although, granted, not impossible). Nissim of Gerona actually says that this statement refers to the majority of women, (Alfasi on Yeb. 6a, end of ד"ה גרסי , beginning on 5b)

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I highly doubt the Gemara would state a generally rule based on probability. Put your example another way: A women plans her wedding to be 15 days after the start of her cycle. With the average cycle being 28 days, that would put her at her peak fertility on the night of the wedding. (The Gemora would probably not create a rule assuming where in her cycle a women MIGHT be holding) Hence, while I don't question the Gemora, using science to explain the Gemora only strengthens the OP's question - as there seems to be very little scientific fact to back it up. –  Mbrevda Dec 29 '11 at 13:26
    
To further clarify my comment: Your scientifically correct that the probability of any random women becoming pregnant is, at best, 1/28 (24 hour window once every 28 days) or 3.57% (100/28 * 1)! Now if the Gemora was using that probability, the Gemora should make a general statement about ALL women: 96+ % of the time a women can't become pregnant! Being that the Gemora is being quite specific about the first cohabitation ONLY, we must deduct that they are NOT using the aforementioned probability. –  Mbrevda Dec 29 '11 at 13:56
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Rashi (Breshit 19:36) points out that Lot's daughters got pregnant on the first act of intercourse even if it doesn't generally happen. He explained that hey were in control and used a special technique. The Chizkuni and Zekenim meba'alei Hatosfot explained that they removed their "betulim".

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Right, but the OP's question is why, according to present-day medical knowledge, that would have been necessary. –  Alex Nov 16 '11 at 21:37
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protected by Isaac Moses Mar 15 '12 at 14:40

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