Some types of fertility treatment in New Zealand may be slightly altering the odds of having a boy or a girl, according to Australian research.

The gender balance for natural births is 105 boys to 100 girls, but some of the techniques used to help would-be parents may lift that as high as 128 boys being born for every 100 girls.

University of New South Wales researcher Professor Michael Chapman, who studied all the assisted reproduction births in New Zealand and Australia, said there was no question of the ratio being inadvertently manipulated by clinicians.

Deliberate sex selection is banned on both sides of the Tasman, unless it is to avoid a gender-specific inherited disease.

Researchers studied all live births following fertility treatment in clinics between 2002 and 2006. Standard in-vitro fertilisation (IVF) was compared with intracytoplasmic sperm injection (ICSI), where the sperm are injected directly into the eggs.

In total, 13,368 babies were born to 13,165 women who underwent the transfer of a single embryo, according to a report of the study in an international obstetrics journal.

The overall male ratio of babies born was 51.3 per cent, but the gender balance varied between IVF regimes: ICSI produced a lower ratio of male babies (50 per cent) and standard IVF a higher ratio (53 per cent).

However, the stage at which the embryo was transferred had more effect, with 49.9 per cent male births from embryos transferred after two days, compared with 54.1 per cent at five days.

And a standard IVF where the embryo was implanted after five days gave 56.1 per cent boys: 128 boys for every 100 girls. But ICSI embryos implanted at two to three days, produced 48.7 per cent male babies: 94 boys for every 100 girls.

Fertility Associates group operations manager John Peek, of Auckland, told NZPA that a check on the firm's own figures for the past eight years, when details of pregnancy outcome were first computerised, "confirms a slight gender difference between IVF (52 per cent boys) and ICSI (48 per cent boys).

He did not yet have enough data to compare the delays before the embryo was implanted.

"Whether a couple has IVF or ICSI is decided by sperm quality," he said. The ICSI injection of single sperm into eggs was to overcome very few sperm or poor sperm quality.

"These changes in sex ratio seen with IVF-related technologies are small, not purposeful, and do not arise from selecting embryos on the basis of gender," Mr Peek said. "It is very unlikely that someone would want to spend $7000 - $10,000 on an IVF-related treatment for a 30-40 per cent chance of any child, knowing that the gender of the child might be 2 per cent greater in the direction desired".

Treatments were used in the conception of about 2.2 per cent of the children born in NZ during 2008, he estimated.