Pregnancy and child

Low birthweight babies catch up by puberty

Babies who gain weight slowly will catch up with their peers by the age of 13, according to the BBC News website. The reassuring report comes with advice from researchers for parents not to over-compensate by increasing their child’s calorie intake.

The news is based on analysis of long-term research, which began in the 1990s, into a host of health issues. The analysis looked at children with poor weight gain during infancy (failure to thrive) and whether this led to long-term problems as a child grew older.

It found that on average, babies who had poor weight gain as infants achieved more-or-less average heights and weights by 13 years. However, their growth was a bit lower than their peers. Babies with later weight gain problems gained weight steadily to the age of 13 years.

Generally, these babies with low-weight problems went on to have slightly lower weight and height as teenagers, compared with teenagers who did not have growth problems in the first nine months of life. Overall, measurements were within the normal range expected for age.

It is worth noting that the study cannot prove that this will be the case for all children, partly because it is unclear how many of the babies received nutritional or medical interventions and due to problems collecting all the data on all of the children. Despite this, the study’s results appear both reliable and reassuring.

Parents may naturally be concerned if their baby is underweight, but this study would suggest low birthweight (at least in the developed world) has little effect in later life.

Where did the story come from?

The study was carried out by researchers from the KP Agricultural University in Pakistan and the University of Bristol in the UK.

One of the researchers received funding from three baby food manufactures: Pfizer Nutrition, Danone, and Plum Baby. As the study did not discuss the potential benefits (or harms) of baby food, this does not appear to represent a conflict of interests.

The original cohort study (The Avon Longitudinal Study of Parents and Children) was funded by the Wellcome Trust, the UK Medical Research Council and the University of Bristol. The study was published in the peer-reviewed journal, Pediatrics.

The story was covered appropriately by BBC News.

What kind of research was this?

This was an analysis of data from a previously conducted large UK cohort study.

The researchers investigated the growth outcomes of children at the age of 13 years. They then compared these in children who had poor weight gain (early or late) in the first nine months of life and those who had normal growth.

Large cohort studies are the best way to follow outcomes over time and are usually designed to be representative of the population (in this case, babies born in the former county of Avon). Cohort studies such as this avoid the bias of relying on referred samples of children with failure to thrive.

However, a practical drawback of cohort studies is that because they often run for long periods of time, follow-up can be difficult for participants and researchers.

What did the research involve?

Researchers used data on 11,499 children who were part of a large Bristol-based study in the 1990s, called The Avon Longitudinal Study of Parents and Children.

Children were excluded from the study if they had a major congenital abnormality likely to affect growth (such as cerebral palsy), were twins or triplets, were born pre-term (less than 37 weeks gestation), or were born post-term (more than 42 weeks gestation), or if any data on them was missing. Babies with extreme weight measurements were also excluded.

To determine weight gain in the first nine months of life, weight measurements were taken at birth, six to eight weeks (which ranged from one to three months) and at nine months (which ranged from six to 12 months).

These weight measurements were then converted into ‘weight scores’ that took into consideration age and gender by using a growth reference chart.

Growth was measured by calculating the difference in the ‘weight scores’ between two time points (birth to eight weeks, and eight weeks to nine months). The researchers adjusted their results for gender, age and the initial weight of the baby when it was born. 

The researchers then looked at the following recorded growth measurements of each included child:

  • weight and height from the age of 9 months up to 13 years
  • body mass index (BMI) at 7, 10 and 13 years
  • mid-arm circumference at 7, 10 and 13 years
  • waist circumference at 7, 10 and 13 years

Babies who had difficulties putting on weight and had weight gain in the lowest 5% of normal weight gains for age were compared to babies who had normal weight gain at each time interval of the study and at 13 years.

What were the basic results?

Of the 11,499 children with weight data available, 507 were considered to have ‘early’ weight gain problems (in the period from birth to eight weeks of age) and 480 were considered to have ‘late’ weight problems (in the period from eight weeks to nine months of age). The main results of this study were:

  • Those identified as having early weight gain problems had improved weight gain in the period eight weeks to two years, and after that gained weight at the same rate as control infants who gained normal weight throughout the first nine months of life.
  • By the age of 13 years, children identified as having early weight gain problems had similar measurements for BMI, arm circumference and waist circumference.
  • The children identified as having late weight gain problems (in the period from eight weeks to nine months) showed a steady, slower weight increase up the age of 13 years.
  • These children with late weight gain problems only had enhanced weight gain compared to the controls in the period between seven and 10 years.
  • The cases identified as having late weight gain problems remained slightly lighter and shorter compared to controls at the age of 13 years.

How did the researchers interpret the results?

The researchers conclude that children with weight problems before eight weeks showed a different pattern of ‘catch up’ compared to babies with weight problems between eight weeks and nine months of age. They say that infants with early weight gain problems ‘caught-up’ in weight at the age of two years, but that height gain was achieved more slowly.

The researchers say that growth outcomes of babies with weight problems were not significantly different to babies considered to have normal weight gain, but that babies with initial weight problems tended to be shorter and lighter.

In discussing the research findings, the lead researcher, Professor Alan Emond, is reported as saying ‘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.’ He added that ‘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.’


Overall, this study provides some evidence that babies considered to have growth problems in the first nine months of life have similar growth measurements at the age of 13 years when compared to baby’s considered not to have growth problems in the first nine months of life.

This study has some limitations worth noting that may limit the overall validity of its findings:

  • The researchers report there was a large amount of missing data, with some outcome measurements only available for 44% of those measured at the age of 13 years. Reassuringly however, there was no difference between missing data between groups.
  • No data is reported for growth up the age of 18, so findings cannot be translated to children above the age of 13, when children are still growing.
  • It is also not known which babies received nutritional or medical interventions if they were identified as having early growth problems, which limits the study findings. 

Despite these limitations, this study provides reassuring news for parents overall.

If you are worried that your child is underweight then seek advice from your GP.

NHS Attribution