Women born very premature, before 32 weeks, are 2.3 cm shorter on average as adults than their sisters born at term, according to a new international study.

The researchers from the Liggins Institute at the University of Auckland and Uppsala University in Sweden were surprised at the finding but don't yet know why it's occurring.

"The 2.3 cm height difference between women born very premature and their adult sisters born at term may not sound much, but to put it into perspective, women born during the Great Chinese Famine in 1959-1961, who experienced severe malnutrition early in life, were about 1.7 cm shorter as adults," says lead author Dr Jose Derraik, a senior research fellow at the Liggins Institute.

There has been 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 the latest of a series of studies by collaborators from the Liggins Institute and Uppsala University.

The researchers have been analysing data from more than 200,000 Swedish women to explore questions about the long-term effects of early life events occurring before, during, and after pregnancy.

The Swedish data were collected between 1991 and 2009 from women aged over 18 years.

The study is published in the American Journal of Epidemiology.