The pitch of new mothers' voices temporarily drops after they have had their first baby, according to a new longitudinal study by Dr Kasia Pisanski, Kavya Bhardwaj, and Prof David Reby at the University of Sussex.

The researchers analysed women's voices over a 10-year period -- five years before and five years after childbirth -- and found that new mothers' voices get lower, and become more monotonous after pregnancy. This 'vocal masculinising' is not caused by aging, as the voice reverts to its previous frequency one year later.

Dr Kasia Pisanski led the research at the University of Sussex's School of Psychology. She said:

"We found that women's voices become lower-pitched and more monotonous after giving birth. One possible explanation is that this is caused by hormone changes after childbirth. Previous research has shown that women's voices can change with fertility, with pitch increasing around the time of ovulation each month, and decreasing following menopause. We know that after pregnancy, there's a sharp drop in the levels of key sex hormones, and that this could influence vocal fold dynamics and vocal control.

"This effect could also be behavioural. Research has already shown that people with low-pitched voices are typically judged to be more competent, mature, and dominant, so it could be that women are modulating their own voices to sound more authoritative, faced with the new challenges of parenting. Additionally, new mums often experience increased mental and physical fatigue, as well as changes in mood and self-perception. This could be reflected in their voices, although given all we know about the impact of hormones and social context on vocal pitch, it's unlikely that this effect is due just to tiredness alone.

"Our results show that, despite some singers noticing that their voices get lower while pregnant, the big drop actually happens after they give birth.

"We analysed voice recordings of natural, free speech during interviews between the mothers and other adults rather than direct speech to their babies, as we know that parents often artificially raise the pitch of their voice when talking to newborns."

The study sample included 20 mothers (and 20 age-matched controls who had never given birth), whose voice recordings were obtained from archives. This allowed the researchers to analyse free speech before, during and after pregnancy. The University of Sussex psychologists analysed 634 interview clips -- equivalent to 277 minutes of audio.

Taking an average, the study found that women's mean voice pitch dropped by over five percent, equivalent to more than one piano note. That's about 15 Hz, or 1.3 semitones. The 'highest' (maximum) pitch of their voices also dropped, by 44 Hz or 2.2 semitones on average. That's more than two piano notes. The researchers also found that there was less variation in voice pitch after childbirth, meaning that the new mothers' voices became both lower pitched, and more monotonous.

Given that a person's voice pitch can affect how they are perceived by others, and can even predict how successful they are in social situations -- from a job interview to a first date -- postpartum changes in women's voices could influence their social interactions. The academics were inspired to undertake this study after hearing anecdotally that the voices of singers and actresses lower after pregnancy. The next step in this line of research is to test whether postpartum voice changes influence listeners' social judgments of new mothers.