Researchers are surprised at their finding that being born very premature appears to have an impact on a girl's adult height.

Researchers at Auckland University's Liggins Institute joined with Swedish counterparts in analysing data from 200 thousand Swedish women.

They found women born before 32 weeks were, on average, 2.3 cm shorter on average as adults than their sisters born at full term.

The researchers don't yet know why.

Lead author Dr José Derraik from Auckland University's Liggins Institute said it may have something to do with many pre-term babies being very thin at birth.

He said one of the study’s collaborators has previously shown that small size at birth is associated with changes in the way growth hormone works in the body, which could eventually reduce adult height.

In New Zealand in 2014, just over 7 per cent of babies were born premature, including 1.3 percent arriving earlier than 32 weeks.