Adele has been hailed as "an artist who comes around once every generation". She is a vocal force whose emotional ballads are masterpieces composed of powerful belts and airy falsettos. For many, it may be hard to imagine that the Grammy Award-winning singer would have trouble nailing any song, especially one of her own.
But during a concert in Sydney last year, Adele had an unusual request for her fans as she prepared to sing her 2012 hit Skyfall, the Oscar-winning theme song from the James Bond movie of the same name.
She asked them to "bear with" her because she was struggling to reach the song's low notes.
"When I wrote that song, I was heavily pregnant," Adele told the audience. She said one of the symptoms she experienced was her voice getting "a lot lower".
"Any other females here sound like a man when they were pregnant? No, just me? OK," the singer said.
Other women have experienced the same phenomenon. And now there's scientific evidence that shows pregnancy and giving birth can cause a temporary change in women's voices.
According to a recently published study conducted by researchers at the University of Sussex, the pitch of new mothers' voices can sound lower and more monotonous after the birth of their first child. The change is described as "vocal masculinising" and could last for at least a year after giving birth.
"Our results demonstrate that pregnancy has a transient and perceptually salient masculinising effect on women's voices," the study said.
Scientists said in a statement that they analysed more than 600 voice recordings from women over a 10-year period, five years before and after they gave birth. They found that a woman's highest pitch dropped by an average of more than two musical notes (about 44 hertz) and her voice also exhibited less variation in pitch. The experiment studied the voices of 20 mothers as well as 20 women who had never given birth. Both groups of women included recordings of public figures, including singers, actresses and journalists.
Two notes, or two adjacent keys on a piano, is a big drop, the study's lead researcher, Kasia Pisanski, told The Washington Post.
"When it comes to human voices, we can perceive pitch differences around five hertz in regular speech, so 44 hertz is a very salient difference," Pisanski said.
While this is the first scientific evidence that having a child affects women's voices, Pisanski said anecdotal claims go back as early as the 1970s. She added that reports mostly came from singers, voice professionals or other women in media, citing Adele as a prominent example.
"For her, it has quite a big consequence in the sense that when she sings songs, they won't sound quite the same," Pisanski said.
Aside from Adele, others, such as actress Kristen Bell, have also spoken about similar experiences while pregnant.
In 2013, Bell described her voice becoming deeper and the change impacting her work on the animated film Frozen.
"There were more womanly tones when I did one recording while I was extremely pregnant," she said. "After I had the baby, I had to go back and rerecord those lines so they matched. There was something different about my voice."
However, Pisanski said her team's research found that the major drop in a woman's voice comes after giving birth rather than during pregnancy. Previous studies largely focused on changes women experienced while pregnant, she said.
The exact cause for this is still unknown, she said, noting that possible explanations include hormone or behaviour changes.
Recent research suggests that the pitch of a woman's voice fluctuates based on fertility. According to a 2008 study, psychologists found that voice pitch increases around ovulation, perhaps to enhance femininity and appear more attractive.
Pisanski said researchers have observed "a big rise in hormones during pregnancy and a drop-off after", which can affect a person's voice.
Additionally, Pisanski pointed to past studies that show people who have lower-pitched voices are judged to be more competent, mature and dominant, which may indicate that changing your voice is more behavioral.
"It could be that women are modulating their own voices to sound more authoritative, faced with the new challenges of parenting," she said.
Pisanski added that the change could also be the result of new mothers experiencing increased mental and physical exhaustion. Given what is known about all the factors that can affect voices, it is "unlikely" that tiredness would be the sole cause, she said.
Whatever the reason, Pisanski said new mothers shouldn't panic if during or after pregnancy they start sounding less like Zooey Deschanel and more like Vin Diesel. The change is temporary, she said.
"I don't think it's anything to be alarmed about," she said. "I'm certainly not sending anyone to a clinic."
The Washington Post