We all know that our bodies change during and after pregnancy, but surprisingly enough, our voices change, too.
While we may associate the arrival of a baby with the high-pitched coos of doting parents, a recently published longitudinal study has found that, in reality, new mums' voices lower in pitch after childbirth when they are talking to fellow adults.
Dr Kasia Pisanski, Kavya Bhardwaj, and Prof David Reby at the University of Sussex analysed women's voices for five years before and five years after childbirth and found that their voices were lower and more monotonous after pregnancy.
Interestingly, though, this 'vocal masculinising' is temporary and not related to ageing. In fact, one year later the mums' voices return to their previous pitch.
Lead researcher Dr Pisanski told Science Daily of the findings, "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 could be a behavioural effect as well, Dr Pisanski noted, with women lowering their voice to fit their new role as a mum.
"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," she explained.
"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."
She said some singers notice a change in their voices, with the pitch dropping during pregnancy. However, Dr Pisanski explained that, in reality, the 'big drop' follows childbirth.
"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," she noted.
In their research, the team assessed audio clips from 20 new mums and 20 of their counterparts who had never given birth. The mothers' mean vocal pitch dropped by more than five percent, which is equal to over one piano note (approximately 15 Hz or 1.3 semitones).
The new mums' highest pitch dropped by more than two piano notes following birth, and their voices overall varied less in pitch. This means their voices became more monotonous.
It makes total sense mums' voices would change postpartum. Vocal pitch can predict how someone is perceived and their success in certain social situations, such as in a job interview.
The next step in this research now involves seeing whether new mums' vocal pitch significantly changes others' perception of them.
We can't wait to see what else they discover! Are you at all surprised by this study's findings?