Children born using in-vitro fertilisation (IVF) are likely to be taller than their naturally conceived counterparts, New Zealand researchers have found.
The study by Auckland University's Liggins Institute found IVF children conceived from fresh, rather than frozen, embryos were about 2.6 centimetres (1.02 inches) taller than non-IVF children by the age of six.
The research, which examined about 200 children, found the height difference was statistically significant, even after adjusting for variables such as the parents' height.
Liggins Institiute director Wayne Cutfield said the phenomenon was most striking in girls.
"At this stage, we don't know what the catalyst for that is," he told Radio NZ on Thursday.
Cutfield said appeared IVF children from fresh embyros had a different hormone profile to regular children, which could promote growth.
This could be caused by the drugs mothers took to induce ovulation during the conception process or by the culture medium the embryos were developed in for 36 hours before being transferred to the womb, he said.
Cutfield said another possibility was that medical workers simply picked the largest, most developed embryos because they were most likely to survive the IVF process.
He said the study showed the need for more research into children born through IVF, which has assisted in up to four million births globally over the past 32 years.
"There's been remarkable little research done on IVF children," he said.
"There's been several studies that have looked at intelligence and cognitive function that havent shown any differences with IVF. This is the first study that's looked very precisely at growth resultation and metabolism."
Cutfield was sceptical about previous studies suggesting IVF children were more prone to conditions such as asthma and arthritis, saying the finding could be explained by other factors.
"Within the IVF cohort, children are more likely to be born particularly small at birth, they are more likely to be born premature, there are more likely to be twins and triplets," he said.
"They're all groups of children who are likely to have greater health problems and you need to be able to adjust for that."
British scientist Bob Edwards, whose pioneering work made possible the birth of the first IVF baby Louise Brown in 1978, was awarded the Nobel Prize for Medicine this month.