Queen

A queen is a warrior she-cat that is expecting or has recently given birth to a litter of kits. Though known to be overly-protective of kits, queens usually are compassionate and kind, and in many cases they have been known to convince their mates or Clan leaders to take in cats in need of help.

Nursing
Queens move to the nursery around a moon before giving birth, surrendering their warrior duties. They have their food delivered to them by their Clanmates as they become less mobile. A she-cat's pregnancy is around two moons, after which she gives birth to a litter of one to five kits.

While kits are weaned at two moons, the queens remain in the nursery until they are named apprentices (typically around six moons) to ensure proper care and education. Sometimes - although it is very rare - a queen leaves the nursery before her kit(s) become apprentices (such as how Squirrelflight did).

When a queen is unable to produce enough milk to feed her kits, another queen who has plenty of milk to spare will help her nurse until the kits are ready for fresh-kill. Queens will also feed kits from outside of the Clan; for example, when Cloudkit was brought to ThunderClan, Brindleface offered to feed him.

A she-cat may request to stay in the nursery after their kit(s) are apprenticed, instead of resuming warrior duties. Goldenflower, Ferncloud and Daisy often helped out when needed in the nursery.

Borage leaves are sometimes given to queens that cannot produce milk for their kits, and parsley for those who still have milk even though they don't have kits.

Restrictions
A medicine cat, either male or female, is not allowed to take a mate and have kits, in order to not be distracted from their mission and work.

An apprentice never has kits, most likely to avoid interrupting their training.

A female leader generally does not have kits either (although it is not explicitly forbidden), because looking after them would keep her away from her leader duties for six moons or more. Male leaders, on the other hand, frequently father kits.

Trivia
In the first books of the Original Series, "queen" seems to mean any she-cat (even female elders are named queens ), not just a nursing cat.