Chinese scientists believe that certain undisclosed nutrients given to embryos during in vitro fertilization may favor the development of male fetuses over female. The study is significant because it could possibly lead to a new technique for gender selection. However, experts are more concerned about what other changes that aren’t as obvious as sex these gender-specific nutrients may bring about.

During IVF a woman is stimulated to produce more eggs, which are then collected and fertilized outside of her body. For around five days fertilized embryos are grown in a petri dish, where lab technicians will then choose the healthiest looking embryos to reintroduce into the body for pregnancy.

Closely Guarded Nutrients

As reported in New Scientist, technicians working on IVF with cows found that the type of culture medium, or nutrients that embryos were grown in could influence how well male and female embryos develop. The 2001 study also revealed that a glucose-rich medium seemed to favor the development of male embryos and the loss of females in cows.

In order to test if the same remained true for human embryos, scientists at Peking University Third Hospital in China analyzed more than 4,400 IVF procedures during 2011 and 2013. The team took note of the type of culture medium used to grow each embryo. Although the specific components of the medium were kept confidential, the team revealed that in one medium the proportion of male births was 56 percent, while for another the proportion was just 45 percent. The uneven proportions remained even when the researchers took other factors into context, such as the parent’s age, body mass, and the type of infertility they were experiencing. Based on these findings, the team concluded that, like the bovine fetuses, human fetuses also varied in their response to different culture mediums depending on their gender.


Gender selection is legal in the United States, but a 2014 survey given by the American Society for Reproductive Medicine revealed that only 21 percent of the American public actually agreed with the practice of choosing gender based only on sex preference with no medical underlying. Also known as gender selection or family balancing, certain techniques can nearly guarantee the birth of a certain sex. However, Slate reported that the American Society for Reproductive Medicine has expressed concern that otherwise healthy women are undergoing unnecessary medical treatments for the sole purpose of gender selection.

“It’s very commonly done,” Dr. David Adamson, a board certified reproductive endocrinologist and surgeon working for the Advanced Reproductive Care fertility clinic in California, told Medical Daily. “The decision that I made not to do that, personally, is not shared by the majority of doctors. I have partners that I respect a lot who will do it. It’s a difficult issue.”

New Scientist also reported that gender selection in IVF is illegal in Canada, the UK, and other Western countries, but this technique presented in the Chinese study could possibly present a loophole. Anti-gender bias laws rule that technicians are not allowed to select embryos based on their gender, but fertility clinics are free to use whichever medium to grow the embryos. That is, if the procedure works — something that experts are still very skeptical of.

“I don’t think it's effective enough to say that it’s a good therapeutic intervention,” Adamson said. “This may well be true, but in the absence of information about the culture medium, it’s difficult to interpret.”

Daniel Brison, a consultant embryologist at the University of Manchester in the UK seemed to agree with Adamson’s reasoning.

"A couple reading this paper who have already got a girl might well say 'hmm, perhaps I should be using the [male-favoring] medium,'" Brison told New Scientist. "But it's not really a very effective method of choosing a gender."

Is It Safe?

Other than the ineffectiveness of the procedure, Adamson raised a far more troubling concern: safety. The period immediately following fertilization is known to be genetically critical, but the culture medium also affects embryo growth. As a study from Maastricht University Medical Center in the Netherlands found last year, culture medium could affect birth weight in babies — a factor tied to lifelong health factors, such as a weak heart and diabetes, New Scientist reported. It’s unknown what other changes adjusting this medium to favor certain genders may result in.

Adamson explained that there is “zero evidence” that such a change would have no other impact other than gender on the embryo. His opinion mounts against many studies that suggest it may. Furthermore, testing necessary to document what effects manipulating the culture media would have on an embryo would be completely unethical.

Source: Zhu J, Zhuang X, Chen L. Effect of embryo culture media on percentage of males at birth. Human Reproduction. 2015.