Freemartins are the 70 percent of females in the World State in Brave New World who are neutered as embryos by being given male hormones. Freemartins don't produce eggs and so can't give birth. Of course, nobody in this society actually gives birth, but freemartins have no eggs to harvest for the creation of embryos and hence zero risk of accidental pregnancy.
Their symbol, a black question mark on a white ground, carries a certain irony. The question mark presumably means their gender is in question or ambiguous because they cannot get pregnant or have children. However, since childbearing has been completely separated from gender in this test-tube baby culture, why that lack would be a mark of gender indeterminacy is not explained. One assumes it must be a holdover from an earlier period when people still equated womanhood with the ability to bear children.
In any case, except for very occasionally sprouting beards and not having eggs, the freemartin are exactly like females: "structurally quite normal" is how Mr. Foster explains it. Though the pregnancy risk is quite low even with fertile women, the freemartins offer men in this culture the opportunity to have sex with absolutely no risk of pregnancy.
As the Controller states: "'You can indulge in any number of pleasant vices with a freemartin.'"