Babies who are slow to gain weight in the first months of their lives generally catch up to their peers by age 13, a large UK study shows.Researchers, writing in the journal Pediatrics, said the results would reassure parents whose babies fail to put on weight quickly.And they warn against boosting the calorie intake of slow-growing babies as this may increase obesity.Experts said that monitoring of weight gain in infants remained vital.The researchers looked at data from 11,499 children who took part in a large study in Bristol in the 1990s.It showed that 507 who were slow to gain weight in the first eight weeks of life recovered fairly quickly and had almost caught up by the age of two years.Another group of 480 children who were slow to gain between eight weeks and nine months continued to put on weight slowly until they were seven years, but then had a spurt and caught up by the age of 13.Normal rangeThe different patterns of recovery between the two groups were likely due to different reasons for slow weight gain, the researchers said.All the children were still lighter and shorter than their peers by the time they were teenagers, but within the normal range.The findings highlight the importance of monitoring a baby's weight and height gain during the first few weeks and months, but not creating anxiety with parents of slow-growing babies, said study leader Prof Alan Emond from the University of Bristol."In the past, a lot of parents have been caused a lot of unnecessary anxiety by health professionals and this is a positive and reassuring message."He said in many cases slow growth where children who are otherwise well do not follow the standard 'curve' is just because they are following their genetic potential."The second point for health professionals is that for a child that is well with no symptoms they can be relaxed and not worry about pushing calories because you can push them the other way."Feeding habits in the second six months of life determine a child's future weight gain, so consuming too many calories in infancy can lead to obesity later on, he explained.Dr Simon Newell, vice-president of the Royal College of Paediatrics and Child Health, said he broadly agreed with the conclusions of the researchers but stressed that poor weight gain was something which needed to be monitored closely."The weight when you're first born is the impact of pregnancy, the weight at eight weeks is related to how well you are feeding, then for the next year there will be an adjustment to your natural centile."He said health professionals needed to look at the whole picture."I would encourage parents to use growth charts but if measurements show your baby is smaller than average it may be completely normal."