Most babies who are slow to put on weight in the first nine months of their life catch up over time and are within a healthy weight range by the age of 13, a new study has found.

Slow weight gain by a baby is often seen as a symptom of an underlying health problem. However, increasing an infant's energy intake can increase their risk of obesity later, therefore doctors are often unsure what to do in such a situation.

Scientists at the University of Bristol in the UK decided to investigate this further. They looked at almost 12,000 people who were born in the 1990s. Of these, 507 were slow to put on weight before the age of eight weeks - these were known as the ‘early group'.

A further 480 were slow to gain weight between eight weeks and nine months - these were known as the ‘late group'.

The study found that children in the early group recovered quickly from their initial slow start and had almost caught up in weight to their peers by the age of two.

Weight gain was different in the late group. They gained weight slowly until the age of seven, then appeared to have a growth spurt between the ages of seven and 10.

"The reason the early group caught up more quickly may be because those infants had obvious feeding difficulties and were more readily identified at the eight-week check, resulting in early treatment leading to a more rapid recovery," the scientists suggested.

By the age of 13, those in the late group were within the healthy weight range, but they still tended to be lighter and shorter than their peers. In fact, by the age of 13, they were an average of 4cm shorter and 12lbs (5.5k) lighter.

Those in the early group were an average of 3.25 cm shorter and over 5lbs (2.5k) lighter than their peers.

According to the scientists, ‘parents can be reassured that well babies showing slow weight gain in the first year do eventually recover to within the normal range, but at 13-years tend to be lighter and smaller than many of their peers'.

They added that the study shows that it is important to monitor a baby's weight and height, but not to make the parents of slow growing babies anxious, as most of these infants will reach a healthy weight range within time.

They also advise doctors not to increase an infant's calorie intake unless ill health requires it, as this may put them at an increased risk of developing obesity later on.

"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 weight problems later in life."

Details of these findings are published in the journal, Pediatrics.