The sources for the 40 days are: Berachos 60a, Nida 30b etc.

You might find the following article interesting:

http://download.yutorah.org/2008...n% 20Science.pdf

Also, it is shitah of many rabbonim (i.e. Rav Kanievsky, IIRC) that in situations where there is a suspicion that there is something wrong with the fetus (and doctors can't help with this), it is better not to take any tests or ultrasound, since this would make to tefillos less likely to be accepted (a nes nigla would be required).
Shimon S | 02.16.09 - 9:29 pm | #

Also, this (40 days) is how we pasken lemaase for tefillah: see Sh"A O"Ch 230:1
Shimon S | 02.16.09 - 9:35 pm | #

I will update teh enrty soon. My med school friedn also showed me his lecture slidees which specifically say that up to about 6 weeks (40 days) the odds of boy or girl are the same.
Reuven Meir | Homepage | 02.19.09 - 11:57 pm | #