From: Rabbi Moshe Sofer
To: Unknown recipient
A community custom has brides recite Shehecheyanu upon their first mikvah immersion before consummation of marriage. The Chasam Sofer defends this practice, arguing that just as a bar mitzvah boy's Shehecheyanu on tefillin covers all his future mitzvos, a bride's Shehecheyanu at tevilah marks her entry into the distinctive women's mitzvos of tevilah, challah, and hadlakas neiros. Communities with this custom should maintain it, but those without it should not adopt it.
Source: Shu"t Chasam Sofer OC §55