'Freemartinism is recognized as one of the most severe forms of sexual abnormality in cattle. This causes infertility in the female calf born twin to a male. When a heifer (female calf) twin shares the uterus with a bull (male calf) fetus, they also share the placental membranes connecting the fetuses with the dam (mother).
A joining of the placental membranes occurs at about the fortieth day of pregnancy, and fluids of the two fetuses are mixed. This causes exchange of blood and antigens carrying characteristics that are unique to each heifers and bulls. When these antigens mix, they affect each other in a way that causes each to develop with some characteristics of the other sex.
Although the male (bull) twin is only affected by reduced fertility, in over ninety percent of the cases, the female (heifer) twin is completely infertile. Because of a transfer of hormones and/or cells, the heifer's reproductive tract is severely underdeveloped, it sometimes contains some elements of a bull's reproductive tract. A freemartin is genetically female, but has many characteristics of a male. The ovaries of the freemartin do not develop correctly, and remain very small. The ovaries of a freemartin do not produce the hormones necessary to induce the behavioral signs of heat.
Freemartinism cannot be prevented, but can be diagnosed in a number of ways. The cattleman can predict the reproductive value of this heifer calf at birth. In some cases, there are no symptoms of freemartinism because the male twin may have been aborted at an earlier stage of gestation.
The estimated percentage of natural beef cattle births that produce twins vary. One estimate puts the percentage at about .5% or 1 in every 200 births. Approximately one-half of the sets of twins should contain both a bull and a heifer calf.'