New child growth charts are being introduced to combat obesity and boost breastfeeding rates.
The UK still uses charts from 1990 which are based on growth rates for formula-fed babies which grow quicker.
The charts, which are being introduced in England with the other nations expected to follow soon, will mean more children are classed as overweight.
But the government hopes it will help to reassure mothers who breastfeed that their babies are not under weight.
Guidelines suggest mothers should give their babies breast milk for the first six months of life and then supplement that with food for a further six months.
 |  In retrospect, I think health staff realise we have probably been worrying some mothers unnecessarily about their baby's weight  Professor Charlotte Wright, of the Royal College of Paediatrics and Child Health |
But breast-fed babies are known to gain weight more slowly during that period and the charts reflect this as by the age of one there is a 1kg difference with the old charts.
But the flip-side of the new system is that the number of babies classed as overweight will double to 6%.
Experts believe this will help identify those at risk of obesity at an earlier stage.
Dr Sheila Shribman, the government's maternity tsar, said: "Breastfeeding is the best form of nutrition for infants.
"The new charts will not only provide more accurate measurements for infant growth of breastfed babies, but will also help healthcare professionals and parents to identify early signs of overweight or obesity and provide support."
And Professor Charlotte Wright, of the Royal College of Paediatrics and Child Health, which helped design the charts, added: "In retrospect, I think health staff realise we have probably been worrying some mothers unnecessarily about their baby's weight."
Healthy children
The charts are based on ones developed by the World Health Organisation after a 15-year study of 8,500 children from across six countries established firm and in-depth data about the growth of healthy children for the first time.
They will be used on all new babies from 11 May with Scotland, Wales and Northern Ireland expected to adopt them in the coming months.
Tam Fry, of the UK National Obesity Forum and chairman of the Child Growth Foundation, said there should have been better training for health workers to help them prepare for the new charts.
However, he welcomed the move, saying the changes should help encourage breastfeeding and "lessen the likelihood of inappropriate referrals to clinics for failure to thrive".
Belinda Phipps, chief executive of the National Childbirth Trust, welcomed the new charts.
She said: "Now that the new charts have been introduced, the next extremely important step is for health professionals to be appropriately trained on how to work with them in order to ensure parents are given the correct information about their child's weight."
Ms Phipps said the previous charts had led to some breastfeeding mothers being advised to use formula milk, or introduce solids as a "top up" because their babies were not gaining weight fast enough.
She said: "The introduction of formula milk or solids too early can interrupt breastfeeding and often brings it to an end earlier."
In England, eight in 10 mothers start breastfeeding but only 22% carry it on until six months. The rates are much lower than other European countries.
