What are freemartins, and what symbol are they given in Brave New World?
I assume that you are talking about the symbols that various kinds of fetuses have on their bottles in Chapter 1. The freemartins are given the symbol of a question mark -- it is a black question mark on a white background.
Freemartins are females who are going to be born sterile. They will not need to use contraceptives (we see later in the book when someone says she wasn't a freemartin and so she had to use contraceptives).
Something like 70% of the female embryos are made into freemartins. They probably don't need that many fertile women, but they want to have a surplus just in case.
In Aldous Huxley’s Brave New World, Freemartins are women that have been intentionally sterilized in order to control breeding. During the sterilization process, an amount of male hormone is injected into the female embryos thereby paving way for a substantial population of infertile females. Freemartins are oblivious of their reproductive condition and even though they retain their female sex organs, they also have secondary male characteristics such as beards. Huxley indicates that in the dystopia, the government’s legislation demands that seventy percent of the female population be freemartins and so only thirty percent of embryos are allowed normal development.
During labeling, while males are represented by the letter T and females by a circle, the symbol for freemartins is a black question mark on a white background.
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.'"