A study which reviewed almost 13,400 babies born to couples who used assisted reproductive technology (ART) has shown how different laboratory techniques carry an unintended gender bias.

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The biggest was seen in cases of IVF (In Vitro Fertilisation) where the embryo had remained in its petrie dish for five days before it was implanted in the uterus.

In 56.1 per cent of these cases, it resulted in a baby boy.

“When you convert that to the sex ratio at birth, that’s around 128 boys to 100 girls – that’s quite significant,” said PhD student Jishan Dean from the University of NSW.

“This is the first time we have used national data, pooling all the clinical data together and looking at the gender of the baby and how the ART procedures can affect this.”

Ms Dean said the cause of the gender bias was not known, though at that time the embryo was a collection of rapidly dividing cells and highly sensitive to its environment.

She said it was not a deliberate effort by IVF clinics to dictate gender – a practice outlawed in Australia except in cases where parents were seeking to avoid passing on a genetic illness.

When an embryo remained in its dish for two to three days before implantation, baby boys accounted for 51.5 per cent or the same rate as seen in regularly conceived births across the community.

The research also assessed results of another more complex IVF technique called Intra Cytoplasmic Sperm Injection (ICSI).

Where the embryo was implanted after five days, the baby boy ratio was a slightly elevated 52.5 per cent. At two to three days it dropped to 48.7 per cent.

“That’s about 95 boys to 100 girls, so less boys,” Ms Dean said.

“This paper is really telling (ART) clinicians that when you were finding more boys and less girls, or more girls and less boys, but … you’re thinking it is by chance, this paper proves it is not by chance.”

The 13,368 babies were born across Australia and New Zealand over the five years to 2006.

Ms Dean said IVF clinics should incorporate the research findings into the information they provide to couples using ART.

IVF laboratory practices should not be allowed to unwittingly contribute to a situation like that emerging in China, she said, where the one-child policy had resulted in many men who could not find a wife.

“It is important that we don’t allow such imbalances to occur unintentionally, simply because we have neglected to study the factors that influence the (gender) ratio in the increasing proportion of the population who use ART,” Ms Dean said.

The research is published in BJOG: An International Journal of Obstetrics and Gynaecology.