A supplement given to breastfed premature babies after they leave hospital can prevent weight loss at a crucial time in their development, a small study suggests.
Doctors in Southampton found eight weeks of extra nutrients led to better growth in babies a year later.
It also gave mothers confidence and encouraged them to continue breastfeeding, they report.
Larger studies are now needed to confirm the findings.
Breast milk alone does not always meet the dietary needs of vulnerable babies born before 37 weeks, with about half failing to grow properly.
Dr Luise Marino, clinical academic paediatric dietician at Southampton Children's Hospital, said all babies' weights dip by about 10% soon after birth, through water loss.
"But you don't want preterm babies to do that," she said.
"They don't have as much fat, minerals or iron, so they need extra nutrients."
Stop too soon
Currently, in the UK, breastfed premature babies are given a supplement packed with proteins and minerals, such as calcium, during their stay on neonatal units.
At this stage, the supplements, also known as breast milk fortifier, are mixed with breast milk and given to babies through feeding tubes.
However, once premature babies are sent home, when they have reached an acceptable weight, the supplements stop.
GPs cannot prescribe them, and so any additional nutrients these babies need are often obtained from formula milk, the researchers from Southampton Children's Hospital said.
They looked at the effects of giving the supplements to 32 mothers and their babies for eight weeks at home and found improvements in the newborns' weight, head growth and length, at eight weeks and again at 12 months, compared with premature babies who were only breastfed.
The average weight of babies in the study was 2.8lb (1.3kg) at birth, and most were born at about 30 weeks of pregnancy.
When they went home, the babies weighed 5.5lb on average.
The supplement sachets can be added to a small amount of expressed breast milk and given to babies by cup or syringe before each breastfeed.
Dr Marino said parents involved in the study said they felt less worried about their baby's growth and found it easy to give the supplements to their babies.
She said larger studies were now needed to see if the results could be repeated.
The study is published in Archives of Disease in Childhood.