There are several festivals in Japan dedicated to the phallus. At the very least, it's only natural that there would also be a religious sites for breasts.
Located in Aichi Prefecture, the temple was established in 1492 and is known as Japan's sole "Breast Temple." However, there is another Kannon temple in Yamaguchi Prefecture called Kawasaki Kannon, which is also centered around breasts.
Kawasaki Kannon isn't nearly as known as Mama Kannon. However, it was recently introduced on So-Net and appeared on Japan Deep earlier this year. As far as I know, there aren't any other temples of this sort in Japan.
Women visit the temple to pray for easy child births and problem-free nursing. And yes, some ask for larger breasts and even pink nipples. (Pink nipples are considered attractive, apparently!)
Typically, when women in Japan are pregnant, they visit either a Buddhist temple or a Shinto shrine to get a haraobi, which is wrapped around the woman's stomach.
And no, most women don't go pray for larger breasts and whatnot when they are pregnant. These two "boob temples" are unique, even in Japan.
Below you can see some images from Mama Kannon:
This is called a "chouzuya" or a "temizuya" (手水舎), and it's where you wash your hands and mouth to purify yourself at a temple or shrine. It looks somewhat different from your typical one...
These are "ema" (絵馬), which are used to write requests on for the gods. I guess these are boob ema.
Yours for 1,500 yen or about US$15.
These are hand-made ema.
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