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A freemartin is defined as a female that shares placental vascular connections with a male twin during fetal development and is sterile as a result of in utero exposure to masculinizing hormones produced by the male. A connection between the two fetal circulatory systems develops early in gestation (anastomosis) and leads to the exchange of blood between the fetuses. As a result of this shared blood circulation, both offspring acquire some of the other's cells, rendering both XX/XY chimeras. Exposure to male hormones leads to underdevelopment of the female twin's reproductive tract. Consequently the female twin (a freemartin) is infertile.
Not all twins share placental vascular connections, so not all females that are born co-twin to males will be freemartins. Additionally, a freemartin may be externally indistinguishable from a fertile female. This test assists owners and breeders to identify suspected freemartins and verify the freemartin status of a female born co-twin to a male.