While breastfeeding, it may look like a baby is chewing on the mother's nipple, but ultrasound images show that the infant actually removes milk by creating a vacuum - also known as sucking.
The finding holds significance as it could explain why some babies fail to take to the breast. It may also shed new light on why breastfeeding really can be a painful experience for some women.
"There have been two theories about how breast milk is expressed," New Scientist quoted Donna Geddes of the University of Western Australia in Crawley, as saying.
"One is that the baby uses a peristaltic or compression motion to actually push the milk out of the nipple and breast. The other theory is that vacuum is primary in removing the milk," Geddes added.
Until now, most studies examining the mechanics of breastfeeding have focused on bottle-feeding infants, or on old X-rays that were of poor quality.
Instead, Geddes and her colleagues combined ultrasound imaging of infants suckling on the breast with measurements of the strength of the vacuum created by the baby's mouth in 20 infants aged 3 to 24 weeks as they breastfed.
"What we see is that when the tongue is lowered and the vacuum is applied, that's when the milk is coming out of the breast, and that doesn't involve any compression of the nipple," Geddes said.
They also found that infants who struggled to breastfeed generated much weaker vacuums than successful breastfeeders.
This may explain why babies with a cleft palate often fail to breastfeed, as do premature babies: preterm infants don't have strong enough mouth muscles to suck hard enough.
According to researchers, the next step is to devise a simple and universal test that could be used to assess babies' ability to suck. This could reassure mothers whose infants are struggling to feed that it's not their fault, they said.
The research was presented at the Medela breastfeeding and lactating symposium in Venice, Italy.