A. Horav Y.D. Harpeness quotes in Chinuch Yisroel (8: 2; p.525) the Kav Hayoshor's assertion that although one is permitted to see unusual non kosher animals for the purpose of reciting a special brocho, still one should avoid continuous and persistent observation, as it brings a spirit of impurity on the observer. He mentions the above question, if a pregnant woman should be more careful and not even engage on a casual look at the zoo animals as it may harm the unborn and remains on a tzorech iyun or that the question requires further research.
Other Poskim are of the opinion that a pregnant woman should avoid going to a zoo. (Migdal Oz - Chalon Tzuri 32. Sefer Hoigud - Hilchos Reiya, and others)
However, others are lenient . (Boinei Beischa page 332, see also Ashrei Yoladeto p. 31: footnote 25).
Horav Shlomo Miller's Shlit'a opinion is similar to Chinuch Yisroel, that a casual look when needed is permitted.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit'a
Important Numbers
