It's explained in the first chapter. Some women are left fertile so the state can take their ovaries and use the eggs to create the next generation.
Still leaning against the incubators he gave them, while the pencils
scurried illegibly across the pages, a brief description of the modern
fertilizing process; spoke first, of course, of its surgical
introduction–"the operation undergone voluntarily for the good of
Society, not to mention the fact that it carries a bonus amounting to
six months' salary"; continued with some account of the technique for
preserving the excised ovary alive and actively developing
It also mentions that a fairly high percentage of women are left fertile as insurance:
"For of course," said Mr. Foster, "in the vast majority of cases,
fertility is merely a nuisance. One fertile ovary in twelve
hundred–that would really be quite sufficient for our purposes. But we
want to have a good choice. And of course one must always have an
enormous margin of safety. So we allow as many as thirty per cent of
the female embryos to develop normally.
For the same reason, some men would also need to be fertile, but in fact men don't seem be sterilised at all, and the contraception described is entirely up to the woman. Possibly a reflection of the times?