Chickens lay eggs and incubate the eggs in a nest. They do not get pregnant. They can lay eggs without mating, but in order for eggs to develop into chicks, they need to mate with a rooster. The mating process is simple and requires only that the birds briefly press their cloacas together in order for the rooster to transfer sperm to the female.
Chickens lay one egg every 25 hours. If the eggs are not collected, the hen lays the eggs in a nest and becomes broody. A broody hen stops laying eggs and instead spends most of her time sitting on the eggs, keeping them warm. She turns the eggs several times a day and keeps them at a constant temperature. If the hen mated with a rooster, the eggs hatch after 21 days of incubation. Chickens attempt to brood unfertilized eggs if the eggs are not collected.
In many commercial hatcheries, the fertilized eggs are collected and artificially incubated. Collecting the eggs induces the chicken to keep laying more eggs, thus maximizing the productivity of the operation. In an artificial incubation setting, the eggs need to be turned regularly and kept at a constant temperature and humidity until the eggs hatch.