Freemartins

A freemartin is a sterile female twin to a male bovine. This condition arises when a female fetus shares a placenta with a male fetus and the placental blood vessels become fused, allowing the exchange of hormones. Typically, the male hormones produced by the male fetus suppress the development of the female's reproductive organs. As a result, freemartins have underdeveloped ovaries and are infertile.

The incidence of freemartinism is high, occurring in approximately 90-95% of twin births in cattle where one twin is male and the other is female. A single-sex twin birth in cattle does not result in a freemartin. Freemartins are often characterized by physical traits that resemble males, such as a coarser coat, a more robust build, and sometimes a slightly enlarged clitoris. Their behavior may also be more assertive and similar to that of a bull.

Diagnosing a freemartin usually involves examining the external genitalia and observing the reproductive tract via ultrasound or at slaughter. While freemartins cannot reproduce, they can be raised for beef production. In some agricultural contexts, they may also be used as recipients for embryo transfer, as their infertile state does not prevent them from carrying a pregnancy to term. The freemartin condition is specific to cattle and is not typically observed in other twin-bearing species.