Parents and doctors worry about prematurity in ­babies. The fear is, if the baby is born before term, that is 40 weeks, their organs aren’t fully developed and they are not fully prepared for ­independent life. The baby may need support in a prem unit.

But how does being born early affect brain development?

In a new study, scientists have shown how the birth weight of babies is linked to cognitive performance at five years of age.

And the results show a heavier birth weight is good for IQ.

We know a combination of weight and how early a baby is born affects later mental ability. This study investigates whether the relative birth weight (weight in conjunction with the length of pregnancy) is a better predictor of later cognitive performance.

Babies whose birth weight is very low are more likely to have some ­developmental problems when they are measured at five years of age.

The study, led by Germany’s Bielefeld University and Warwick University, combined data on 30,000 children from around the world. Traditionally, researchers have used the 10th “percentile” as a potential risk factor affecting long-term development. So what does percentile mean?

It compares a baby’s weight to that of other babies the same gender, weight and age. So if a baby weighs in at the 90th percentile it means 90% of babies weigh less. If a baby is in the 10th percentile for weight, only 10% of babies of the same age weigh less, and 90% weigh more.

Lead author Dr Robert Eves says the 10th percentile is a questionable cut-off point and the connection between birth weight and cognitive ­development is more gradual. The ­scientists also wanted to see if there were problems with a very high birth weight.

Turns out, using just the 10th percentile oversimplifies things. Increasing birth weight percentiles are linked to increasing cognitive scores, from the first percentile up to the 69th percentile and then it plateaus out.

So effectively there’s no cognitive difference between babies born at the 69th, 79th, or 89th percentile.

Dr Eves added: “For example, we find the estimated cognitive performance of an infant born at 30 weeks gestation and at the 69th birth weight percentile to be similar to that of an infant born at 35 weeks gestation and the 2nd birth weight percentile. Thus, a more nuanced approach may be beneficial.”

He also emphasises while relative birth weight and gestational age may influence childhood development, there are many other factors, such as parent-child relationship, that help to shape the development of the child.

Reassuring for parents.