Not only are male foetuses statistically more likely to be miscarried, but it is also known that fewer males are likely to be born during times of great stress, for example during the occurrence of natural or economic disasters. So why do male foetuses seem to have it so much tougher?
The research carried out by Ole Christiansen, a consultant registrar at the Rigshospitalet Fertility Clinic in Copenhagen, investigates the relationship between gender and the outcome of pregnancy. His study examined patients who suffered secondary recurrent miscarriage – the loss of at least three pregnancies to miscarriage following one successful pregnancy. Christiansen collected the data from 204 patients admitted to clinics in Denmark between 1986 and 2000, and then obtained information on subsequent pregnancy outcomes in 181 patients.
Of the women who had a girl in their first pregnancy, 73% had given birth to a second live baby by January 2002, compared to the much lower 54.4% of women whose first child was male. Christiansen suggests that this might be caused by the way in which women’s immune systems react to male foetuses. Paternal genes have significant influence on placental growth and function, and the type of tissue which forms the surface of the placenta. Christiansen explains “The placenta is created from the foetus, and if it is a boy it will carry these male-specific tissue types. The mother’s immune system may be reacting by forming antibodies, but also the mother’s white blood cells may be reacting against the placenta.”
However, first pregnancies involving a male foetus are less likely to result in a miscarriage because the foetus has time to become stronger and more securely established, before the mother’s immune system can react to male tissues. The problem becomes significant during later pregnancies, as the immune system may remain activated after delivery, and could affect subsequent male foetuses.
Christiansen’s outlook is hopeful: “We believe that our research will be able to clarify whether these complications may be related to immunization against male-specific antigens. If this turns out to be the case, then I believe that we already have a quite efficient treatment.”
But the risks to male foetuses extend further: the proportion of newborn boys often dips sharply during times of crisis. A group of researchers at the University of California have been studying the impact of the 9/11 tragedy on male babies since 2005, and their findings are revealing. Using the data from the National Vital Statistics System, they collected information of all foetal miscarriages in the country between 1996 and 2002, covering over 156,000 foetal deaths.1 During the September of 2001, the death rate of male foetuses compared with female increased by 12%. The true of impact of the events of 9/11 may be even more startling, as only 10-30% of miscarriages are reported. Tim Bruckner, the lead author of the study indicates towards this underestimate: “The magnitude is reflected in our data, but that 12 percent likely applies to a much larger number”
One explanation as to why such a disproportionate number of male foetuses are miscarried, which is supported by researchers from the National Academy of Sciences suggests that the mother’s body under stress has less tolerance for weaker, unhealthy embryos and foetuses. Dr Ralph Catalano, lead researcher of a University of California study examining data on Swedish births from 1751 to 1912, suggests that this is a genetic strategy. Their research suggests that with only weak foetuses being selectively targeted by the mother’s body, the stronger males are given greater opportunity to thrive, and so arguably the chance of a woman’s genetic line surviving was increased as stronger males would go on to produce more offspring. Thus, aborting boys may actually maximise the odds of survival of the bloodline. Dr Catalano concluded: “These findings demonstrate yet again that we need not go to museums of natural history to find evidence of natural selection.”
So could we see the gender ratio skew as a result of the recession? Bruckner seems to indicate that this would be a “logical extension” of his team’s findings.
Maybe men really do have it that little bit tougher than us girls….