Babies born into larger families are less likely to thrive, research suggests.
Good early growth is very important
A Bristol University team found these babies were more likely to struggle to gain weight in their first nine months.
However, the problem seems to have nothing to do with the prosperity of parents - the effect was found across all social classes.
The findings come from the Children of the 90s project following the progress of 11,700 infants.
Failure to thrive is a medical term used to describe infants and young children
whose growth is substantially less than that of their peers.
Risk of failing to thrive
First born - 3.4%
Second born - 4.6%
Third born - 5.8%
Fourth born - 8.3%
Failure to thrive is defined as being in the bottom 5% of weight gainers
There is evidence that it is associated with a delay in proper development, and possibly problems in later life, including intellectual deficits.
The babies in the study were weighed at birth, at six to eight weeks and at nine months.
The researchers then focused on the 5% of children who put on least weight over that time.
Tall parents an advantage
It had been suggested that low socio-economic status was associated with poor growth.
But this was not borne out by the study, which found babies born to unskilled, manual parents were no more likely to be adversely affected than babies born to professional people.
However, the study did find evidence that babies born into larger families were more likely to fail to thrive.
Even a mother's second or third baby was more likely to fall into this group, and a fourth child was at twice the average risk of being among the slowest growers.
The research also uncovered a strong correlation between parental height and slow weight gain up to nine months.
Babies of short parents were eight times more likely to grow slowly when compared to babies with tall parents.
Lead researcher Dr Peter Blair the lead author said the reasons why babies in large families grow more slowly were not clear, and more research was required to try to pin down a reason.
However, it is suspected that the problem may be linked to hard-pressed parents finding it impossible to attend carefully to the needs of their younger children when they have a big family.
He added: "While it may not be surprising that babies of short parents are more likely to grow slowly - we are the first study to establish this association.
"Growth standards for future measurement need updating and parental height must be part of this new calculation."
The project is now investigating the growth and development of the children at seven and eight to assess the longer-term consequences of poor growth in the first year of life.