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I have read reports of the accuracy of gender prediction from an ultrasound with numbers ranging from 90-98%.

There is a halacha (Y.D. 194:11) that if a fetus sticks its hand [or possibly any limb] out of the womb, and then retracts it, and isn't born for another two weeks, then the mother takes on the "impure" status of a yoledes (woman who gave birth). This status lasts for two weeks, because we don't know if the fetus is a male or a female and we must take into account that if it is a female, the "impurity" is two weeks, as opposed to the one week of impurity for a male.

However, if we would know that the fetus is a male, then the impurity would only be one week.

If an ultrasound was performed, and the gender of the child was determined to be male, could this be relied upon to limit the "impurity" to one week?

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    I believe Rav Moshe Feinstein used similar reasoning to ignore vestos after a positive pregnancy test, so it would seem to follow logically rather well. – Loewian Jan 11 '16 at 5:46
  • Even better, what about karyotyping a sandal? – Double AA Jan 11 '16 at 12:37
  • @DoubleAA what is karyotyping? – jim Jan 17 '16 at 17:23
  • @jim en.wikipedia.org/wiki/Karyotype – Double AA Jan 17 '16 at 22:42
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Rav Moshe Feinstein, z"l, rules (Igroth Moshe YD 3:52) that a pregnancy test ("בדיקות הידועות לרופאים") can be relied upon to assume halachically that a woman is pregnant (with regard to whether she would need to continue to keep w'sathoth). It would seem to logically follow that, provided the results of the ultrasound were as reliable as such a pregnancy test, they would also qualify as a halachic proof for tumah/tahara purposes. (See also here.)

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    We generally assume vestot are derabanan while Tumat Leida is Deoraita. I'm not sure you can compare the two. – Double AA Feb 3 '16 at 5:14
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    @DoubleAA I hear your point. Unless we view a safek d'orayta as an (equivalent) d'rabanan... – Loewian Feb 3 '16 at 5:21
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In modern times as a woman who has given birth to seven children; if a fetus sticks any part of it's body out of the womb a doctor will likely deliver the baby as soon as possible. Babies don't go back in for a week or two after the waters break which must happen in order for any part of the child's body to "stick out" .

Applying simple logic to the question if somehow it were to happen and a woman were to be impure as though she had given birth though; ultrasounds can't be 100% reliable, 3D ultrasounds maybe. Today doctors aren't always able to determine gender. Sometimes children are born with intersex conditions. There was a case where the external appearance was male and the internal organs were female.

My answer would be to be safe rather than sorry. Assume that you are impure from the time the limb comes out of the womb until after the baby is BORN. Determine the sex as best you can at birth and follow through with the appropriate time period.

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