See this article: theconversation.com/pregnancy-temporarily-lowers-a-womans-voice-new-study-97424
"We found that mothers’ mean and minimum pitch dropped 14 Hz on average (around 1.3 semitones) after pregnancy compared to before. The maximum voice pitch of new mothers dropped as well, by 44 Hz or 2.2 semitones on average, confirming that reaching high frequencies may be particularly challenging for some women in the year following childbirth. New mums also had more monotone voices." They're not sure if it's hormones or some biological instinct that kicks in.
Did anyone find this?