Freemartinism is an intersex condition in cattle that occurs when a female twin shares blood supply with a male twin in utero. This allows masculinizing molecules from the male twin to disrupt the female's sexual development, preventing fertility. A freemartin is defined as a sterile female calf born as a twin with a male due to placental vascular anastomoses between fetuses of different sex, leading to disrupted differentiation of the female's ovaries and genital tract. Diagnosis can be made through physical exam or serological/cytogenetic testing, and early detection is important to cull infertile freemartin heifers from the herd.