A baby can hear by the third trimester of pregnancy
It has long been known that a baby can recognise its mother's voice shortly after birth.
But now new research goes further - it seems a baby responds to its mother when it is still in the womb.
Hearing her voice is enough to start its heart racing, scientists in Canada have found.
This may help the unborn child learn and develop language ability as well as helping the bonding process.
It was discovered a few years ago that foetuses can hear by the third trimester of pregnancy.
The researcher, Dr Barbara Kisilevsky, has now found that full-term infants in the womb respond to their mother's voice.
She came to this conclusion by carrying out a novel experiment with colleagues in China.
Audiotapes were played to 60 pregnant women to see how the developing child responded to the voice of either its mother or a female stranger.
The scientists found the baby's heart-rate speeded up when it heard its mother but slowed down in response to a stranger's voice.
Dr Kisilevsky, a professor of nursing at Queen's University, Kingston, Ontario, believes a baby is capable of learning and remembering even before birth.
She said: "These results tell us that the foetuses heard and responded to both voices and that there was sustained attention to both voices.
"But, because they responded differently to the two voices, we know they had to recognise their own mother's voice.
"We believe they are probably already learning about language in general and their own language specifically."
The experience of a child in the womb is thought to play a role in both development and mother-infant attachment.
Mary Nolan, an antenatal teacher with the National Childbirth Trust in the UK, said women are encouraged to sing, read poetry or play music to their unborn child to enhance bonding.
"It's building up the bond and developing the baby's ability to perceive different voices and tones and intonations," she said.
"It may be that after the birth these particular sounds when played to him again are very soothing."
The research, which is published in the journal Psychological Science, was carried out in collaboration with obstetricians in Hangzhou, China.
The next step is to look at how an unborn child responds to its father's voice and whether it can distinguish between English and Mandarin.