The paper explains, According to this maternal immune hypothesis, cells (or cell fragments) from male fetuses enter the maternal circulation during childbirth

The paper explains, According to this maternal immune hypothesis, cells (or cell fragments) from male fetuses enter the maternal circulation during childbirth or perhaps earlier in pregnancy. These cells include substances that occur only on the surfaces of male cells, primarily male brain cells. The mother’s immune system recognizes these male-specific molecules as foreign and produces antibodies to them. When the mother later becomes pregnant with another male fetus, her antibodies cross the placental barrier and enter the fetal brain. Once in the brain, these antibodies bind to male-specific molecules on the surface of neurons. This prevents these neurons from‘‘wiring-up’’in the male-typical pattern, so that the individual will later be attracted to men rather than women.