"Historically, contraception was believed to affect the voice negatively. However, more recent studies using low-dose oral contraceptive pills (OCPs) show that they stabilize the voice. However, stabilization generally occurs only during sustained vowel production; connected speech appears unaffected. Therefore, singers may be the only population that experiences clinically increased vocal stability as a result of taking hormonal contraceptives."
January 1, 1970
https://en.wikiquote.org/wiki/Hormonal_birth_control