From a University of Sussex study:
Over 600 voice recordings were collected from 20 mothers and 20 age-matched controls (“nulliparous” women who had never given birth). Both samples included singers, actresses, journalists and reporters. These voice clips were then acoustically analysed using Praat, a popular open-source acoustic analysis program for measuring human speech.
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…
Read on here.