It was something the female characters used in the book so that way they would not get pregnant.
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I don't believe we're given the name of the borth control, although we are told how it was carried.
And round her waist she wore a silver-mounted green morocco-surrogate cartridge belt, bulging (for Lenina was not a freemartin) with the regulation supply of contraceptives.
"The discoveries of Pﬁtzner and Kawaguchi were at last made use of. An intensive propaganda against viviparous reproduction ."
"Perfect!" cried Fanny enthusiastically. She could never resist Lenina’s charm for long. "And what a perfectly sweet Malthusian belt!"
Brave New World