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 about 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 just over two moons, after which she gives birth to the possible 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 Moonflower did). Also, sometimes they leave before all of their kits become apprentices, such as Rainflower.

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 such as Daisy and Ferncloud did when Squirrelflight didn't have enough milk. Queens will also feed kits from outside of the Clan; for example, when Cloudkit was brought to ThunderClan, Frostfur convinced Brindleface to feed him.

A she-cat may request to stay in the nursery after her 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. Leafpool and Yellowfang were two examples of breaking this rule.

An apprentice never has kits, most likely to avoid interrupting their training, and a result of being too young.

A female leader or deputy generally does not have kits either (although it is not explicitly forbidden, as Leafstar had kits), because looking after them would keep her away from her leader or deputy duties for six moons or more. Male leaders, on the other hand, frequently father a she-cat's kits, such as Hailstar, Pinestar and Firestar.

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.