Originally Posted by
SJUAMMF
If sentos are closing in inner cities because traditional houses are disappearing, then it will be doubly hard to find one offering lower temperature for pregnant ladies who still live in those few traditional houses? May be obasans?
In any case, using the search term "東京 微温泉", here is one in Nishi Shinjuku. Which should be close enough to where you will be and 25.9 deg C should be pretty cool.
http://www.juken-net.com/magajin/maga/79.htm
BTW, it seems this onsen is naturally at this temperature, not a consideration to pregnant clients.
I'll bet it isn't! That's 1,900yen (nearly £14) for a dip

Thanks (and well done!) for finding this. Shame it's priced well above my means.
Originally Posted by
SJUAMMF
Human body actually regulate internal temperature to a pretty constant level regardless of the ambient temperature. The temperature caution should be the fact that bather may be a little dizzy come out of the onsen after fighting the elevated temperature and not the fact that internal temperature is elevated. Only sick people have elevated internal temperature.
But I guess you are not concern with what other people are concerned about such as gems and slippery floors in onsen/sento.
Germs and slippery floors do not particularly concern me, just as they don't concern other women who keep fit in pregnancy by swimming. Now gems on the floor is actually something I've come across and does concern me. Happily, I can feel confident that the gem, once handed in, will be returned to it's owner
Apart from the elevated risks of dehydration and hypotension for pregnant women, there are links between maternal hyperthermia and fetal malformation. The risks might be low, but this is not something I want to gamble on.
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I just called the Shibuya Ward and lodged my enquiry with them. I've arranged to call back next Friday so that they have time to look into this for me.
I'll post back when I find out some more.
Thanks again,
mjm, for the lead.