Transfer of more than one embryo per cycle
Apart from the possible health and, in many cases, subsequent social complications associated with multiple pregnancies, one very good argument against embryo transfer of more than one embryo in one cycle was added. In the case of twins there is a 50% chance of one being a girl and the second a boy. A study of 13,800 twins, born 1967 to 1978 in Norway, published last week in the prestigious PNAS magazine, shows that girls who are born of such pregnancies are life-disabled.
They are more likely to have behavioral disorders with a tendency to aggressiveness, to achieve worse results during their studies and less of them complete secondary or higher education. In this context, they are less successful in life, have lower incomes and a lower standard of living. They are also more likely to experience reproductive problems when they reach maturity.
The blame is the exposure of testosterone, which is produced by a male sibling during intrauterine development. For this hormone, the placenta does not represent an effective barrier and adversely affects the development of the female brain. A similar effect of testosterone during intrauterine development on the female fetus was described about 50 years ago in some mammalian species, leading mostly to complete female sterility.
(Original text: https://www.pnas.org/content/116/14/6749)