Battles between the sexes have been waged endlessly, but a study gave a definite edge to females. Researchers at the Helen Schneider Hospital for Women and the Sackler School of Medicine at Tel Aviv University in Israel concluded that outcomes for twin pregnancy are enhanced when at least one of the twins is a girl. The study evaluated more than 2, twin pregnancies and contrasted the outcomes for girl-girl, boy-girl, and boy-boy twins. The researchers found that the incidences of preterm deliveries were higher in boy-boy twins, and the babies had lower average birth weights and lower growth rates when both twins were male. Meanwhile, girl twins had fewer respiratory and neurological problems. Interestingly, the results showed that it only took one girl to improve the outcome for a boy; across the board, boys with a twin sister fared better than sets of boy twins.
Female twins who shared a womb with a brother tend to get less education, earn less money, and have fewer children than girls who shared a womb with another girl, according to an analysis of hundreds of thousands of births over more than a decade. Researchers suspect the cause is testosterone exposure during fetal development, though the exact mechanism remains a mystery. Still, she cautions, a lot more work needs to be done to establish a causal link. Fraternal twins, in which each of two eggs is fertilized by a different sperm cell, occur in about four of every births. About half of those result in male-female twin pairs.
Girls with a male twin are less likely to thrive due to testosterone exposure, report suggests
Girls with fraternal male twins may earn less and be less likely to thrive in society, new research suggests. Researchers in Norway and at Northwestern University posit that exposure to their brother's testosterone in the womb may damage the female twin, resulting in poorer cognitive functioning and fertility. Though studies on humans vary, it may be that this testosterone exposure alters the way females develop, leading to more masculine traits that could explain the lower probability that these girls find partners and marry later in life. Understanding these outcomes is more important now than ever, the researchers say, as IVF has let to a near two-fold increase in the number of twins born annually.
Is this really the case? And how does it work? Between week 8 and 24 of pregnancy, a male fetus is exposed to testosterone, whereas a female fetus is not. That is to say, it enhances the development of typically male traits. For example, increased aggression has been attributed to relatively high prenatal testosterone exposure.