Monday 18 October 2010
How IVF 'skews the ratio of boys to girls': Male embryos appear better equipped to survive
Certain types of IVF treatment may increase the odds of having a boy, research revealed last month.
Some methods tip the gender balance to as many as 128 boys being born for every 100 girls. The balance for natural births is 105 boys to 100 girls.
The discovery was made by Australian researchers, who believe that the IVF process may be affecting the sex ratio.
Certain types of IVF treatment may increase the odds of having a boy, research has revealed.
Although the reasons remain unclear, the researchers suggested it could be because male embryos may in some way be better equipped to survive the process.
They warned more research is needed to ensure a serious imbalance of male to female children does not develop as IVF becomes increasingly common
Study leader Professor Michael Chapman, from the University of New South Wales, said there was no question of the ratio being manipulated by IVF clinicians. Deliberate sex selection is banned in Australia, as it is in Britain except in very specific circumstances, such as to avoid gender-related hereditary disease.
Professor Chapman said laboratory methods may be responsible for the imbalance, particularly the substance in which embryos develop in the test tube.
‘It could be that fitter embryos are male and it may be the female embryos which fall away at the various hurdles,’ he added.
Some methods tip the gender balance to as many as 128 boys being born for every 100 girls.
‘If we can discover which techniques are responsible for the difference, and why, we may be able to ensure the sex ratio returns to normal.’
Researchers studied all live births following fertility treatment in clinics in Australia and New Zealand between 2002 and 2006. Standard IVF was compared with intracytoplasmic sperm injection (ICSI), an IVF variant where the sperm are injected directly into the eggs, normally because the sperm may be too weak or few in number to penetrate the egg naturally.
They also took into account exactly when the embryo was implanted into the womb – either two to three days after fertilisation when the egg splits into two cells, or after five days, when it has developed into up to 100 cells in the lab.
In total, 13,368 babies were born to 13,165 women who underwent the transfer of a single embryo, says the study, reported in BJOG: An International Journal of Obstetrics and Gynaecology.
The overall male ratio of babies born was 51.3 per cent, not far off the figure of 51.5 per cent for the general Australian population from natural conception.
However, the study found specific IVF regimes changed the balance.
Compared with natural conception, ICSI produced a lower ratio of male babies (50 per cent) and standard IVF a higher ratio (53 per cent).
The stage at which the embryo was transferred had even more effect, with 49.9 per cent male births from embryos transferred after two days, compared with 54.1 per cent at five days.
However, the combination of the two factors led to the greatest alteration in sex ratio.
After standard IVF where the embryo was implanted after five days, 56.1 per cent of births were boys. This represents 128 boys for every 100 girls.
But when ICSI embryos were implanted at two to three days, 48.7 per cent of babies were male – 94 boys for every 100 girls.
Professor Philip Steer, BJOG editor-in-chief, said the use of IVF to help childless couples may have health and social implications.
In 2007, the latest year for which there are official figures, there were more than 13,000 babies born through IVF in the UK – around 1.5 per cent of all births.