In most cattle twins, the blood vessels in the chorions become interconnected, allowing blood from each twin to flow around the other. If both fetuses are the same sex this is of no significance, but if they are different, male hormones pass from the male twin to the female twin. The male hormones then masculinize the female twin, and the result is a freemartin. The degree of masculinization is greater if the fusion occurs earlier in the pregnancy – in about ten percent of cases no fusion takes place and the female remains fertile.
The male twin is largely unaffected by the fusion, although the size of the testicles may be slightly reduced.
Testicle size is associated with fertility, so there may be some reduction in bull fertility.
Freemartins behave and grow in a similar way to castrated male cattle (steers).
Genetic testing for the Y-chromosome can be performed within days of birth and can aid in the early identification of a sterile female bovine.
Physical examination of the calf may also reveal differences: many (but not all) freemartins have a short vagina compared with that of a fertile heifer.
