The voices of new mothers temporarily drop by more than one piano note after pregnancy, scientists have claimed.
Pop star Adele said she sounded "like a man" while expecting, which helped her to hit the low notes in Oscar-winning James Bond theme song Skyfall, but new research suggests she is not alone in experiencing a vocal change.
Researchers from the University of Sussex found new mothers' voices become lower and more monotonous after they have had their first baby.
However, this "vocal masculinising" does not appear permanent, the team said - with voices reverting to previous frequency after one year.
They speculate the change may be an attempt to sound more authoritative in response to the new role of parent, or could be driven by hormones.
Lead researcher Dr Kasia Pisanski, of the University of Sussex's School of Psychology, said: "Our results show that, despite some singers noticing that their voices get lower while pregnant, the big drop actually happens after they give birth."
The study examined 20 mothers, including singers, actresses, journalists and celebrities, and compared them to a control group.
Analysis of archive interview footage in the five years before and after pregnancy found women's mean voice pitch dropped by over 5% - the equivalent to around 1.3 semitones.
The women's highest pitch also dropped by an average of 2.2 semitones, or more than two piano notes, and they had less variation in pitch, the study found.
"Our results demonstrate that pregnancy has a transient and perceptually salient masculinising effect on women's voices," the authors said.
Dr Pisanski added: "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."
She added that it was "unlikely" the effect was due to mental and physical fatigue alone.
Adele previously said she now struggles to hit the lowest notes in hit Skyfall.
Last year she told a crowd in Sydney, Australia: "When I wrote that song, I was heavily pregnant.
"And a side effect or symptom, however you felt about your pregnancy, was my voice got a lot lower, so my larynx dropped.
"Any other females here sound like a man when they were pregnant? No, just me? OK.
"Basically, that's why the verse is so very low. That's the reason, and these days I do struggle to get down there, so do bear with me, alright?"
Kristen Bell, who voiced lead character Anna in Disney hit Frozen, reportedly had to to re-record lines in the film because pregnancy made her voice deeper.