Babies born very premature tend to grow up significantly shorter than those who go full term in pregnancy, a new Kiwi-Swedish study indicates.
The study of more than 200,000 women, by researchers from the universities of Auckland and Uppsala, Sweden, found those born before the 32-week point in pregnancy were nearly three times more likely to grow into short adults than those who go the full term of 37 to 41 weeks.
The very-premature babies were also found to be 2.3cm shorter in adulthood than their full-term sisters, although the difference shrank to 2cm when the mother's stature was considered.
It may be related to the fact the premature babies are often quite thin at birth
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The average heights of Kiwi adults aged 20 to 49, at 164cm for women and 177cm for men, puts us close to the OECD average. Swedes in their 20s are taller, at 166.8cm for women and 181.5cm for men.
The very-premature Swedes in the study were on average 165.5cm, compared with 167.3cm for the full-term women, a difference of 1.7cm.
Writing in the American Journal of Epidemiology, the researchers say that, similar to their own findings, a study of Swedish men found that those born very premature had a nearly 3-fold higher chance of being short - less than 166.3cm - as an adult.
But the authors say it is not clear if their findings on Swedish women are applicable to other ethnicities.
In New Zealand, around 4400 babies, 7 per cent, a year are born premature - including 750, or 1.3 per cent, who are very premature. Worldwide, around 10 per cent of babies are born premature.
"We were a little surprised by the results," said the lead author, Dr José Derraik, of the Liggins Institute at Auckland University.
"There is some evidence that babies who were born premature tend to be shorter in childhood, but they usually catch up with those born at term in late adolescence.
"But our study shows that women who were born very preterm fail to reach the stature you'd expect based on their parents' and siblings' heights."
"This is one of the very few studies that has specifically investigated the association between premature birth and adult height.
"Also, we believe this is the first study to examine this association between adult siblings."
Comparing siblings allowed the researchers to control for possible genetic effects on height.
The researchers do not yet know why premature birth is related to shorter adult stature.
"It may be related to the fact the premature babies are often quite thin at birth," Derraik said.
A Liggins co-researcher on the study, Professor Wayne Cutfield, has previously shown that small size at birth is associated with changes in the way growth hormone works in the body, which could reduce adult height.
In the journal paper, the researchers compare their findings with women born during the 1959-61 Chinese famine who suffered severe malnutrition in early childhood and were about 1.7cm shorter in adulthood.