A Pomeranian is in heat when she displays a swollen vulva, a strong sex drive, and several other physical and behavioral changes. Also known as estrus, heat involves sharp swings in hormonal levels that prepare the dog's body for mating and pregnancy.
Other physical changes that indicate heat are swollen teats and vaginal discharge that varies from red to pink to yellow over the course of the heat. Behavioral changes include differences in sociability, frequent grooming of the vulva and a "nesting instinct" often indicated by hoarding toys.
First heat in a Pomeranian is, on average, at 6 months of age but may happen as early as 4 months. Smaller dogs tend to start undergoing heat earlier, and most Pomeranians undergo their first heats before they are 1 year old.