7.5. Freemartinism

The double ovulation occurs when two mature follicles are released at the same time in one cycle of ovulation [21]. Adhesion of the outer layers of the amniotic sacs can occur in twin and multiple pregnancies because of their proximity. This may result in fusion of the chorion layers at which point an anastomosis of the vessels occurs in most cases leading to a union of the blood circulations of both embryo. In case of twins with different sex (bull and heifer), in 98% of all occurrences, heifers are not suitable for breeding because of freemartinism [67], causing infertility in the female calf born with a male twin. As fertile heifer calves are far more valuable than bull calves, this is an issue on dairy farms.
