The nutrient-filled liquid used to grow embryos during IVF might affect the resulting male to female birth ratio, a study suggests. But this only seems to be true when the sperm is injected directly into the egg during fertility treatment (a technique known as ICSI).
The paper, published in Human Reproduction, analysed the relationship between sex at birth and culture medium during IVF using a sample of 4,411 births at Peking University Third Hospital between 2011 and 2013.
Lead author Professor Jinliang Zhu and colleagues showed a significant variation in the proportion of males at birth between four common IVF culture media, which was independent of other factors such as parents' age or BMI.
Yet the trends only held true when ICSI had been used during treatment. For those births, one medium resulted in 56 percent males at birth, another only 45 percent. Unfortunately, the precise ingredients of each culture medium are tightly guarded commercial secrets so the scientists could not advance very far in their analysis.
According to New Scientist, which broke the story, 'Zhu's team think that the male and female embryos respond differently to the mix of ingredients the different manufacturers use. It is already known that there are metabolic differences between male and female embryos - male embryos develop faster than female embryos, for example.'
Dr Roger Sturmey, a senior lecturer in Reproductive medicine at The Hull York Medical School, told the magazine the result was 'a highly significant finding'.
Meanwhile, Professor Allan Pacey, at the University of Sheffield, urged caution in interpreting the result: 'It could be down to the selection of sperm, embryo development rates, how the embryos implant, or another mechanism related to miscarriage.'
However, New Scientist points out that previous studies have shown that choice of embryo culture medium can apparently influence post-natal weight in the resulting babies, with this effect lasting for up to two years.
If the medium could also influence the baby's sex then fertility clinics might gain a loophole to escape the anti-sex-selection legislation in many countries, including the UK; IVF practitioners are not allowed to select embryos for implantation based on sex for non medical reasons, but can decide which culture medium is used during treatment.
However, Dr David Adamson, a reproductive endocrinologist and surgeon at the Advanced Reproductive Care Fertility Clinic in California, USA, told Medical Daily: 'I don't think it's effective enough to say that it's a good therapeutic intervention. This may well be true, but in the absence of information about the culture medium, it's difficult to interpret.'
Speaking to New Scientist, Professor Daniel Brison, a consultant embryologist at the University of Manchester, echoed his thoughts. 'It's not really a very effective method of choosing a gender,' he said.