The 29-year-old pop star helped deliver a baby - at home.
"Finally you can add 'helps delivers babies in living rooms' to my resume! It's been a miracle of a day … Auntie Katy aka Stylist Auntie," Perry tweeted to her 50 million followers on Wednesday.
Though she did not reveal the mother's identity, it's believed to be her older sister Angela Hudson.
Perry's exciting news comes amid reports that she and Mayer have broken up.
Reps for the couple have not responded to requests for comment, and neither Perry nor Mayer, 36, has spoken about the breakup. However, the pair have not been seen together in recent weeks.