Average cycle: every 6 months (range 4-12 months)
Female dogs cycle approximately every 6 months (range 4-12, breed-dependent). The cycle has 4 stages: Proestrus (7-10 days) — vulva swelling, bloody discharge, attracts males but won't mate. Estrus (5-14 days) — fertile period, discharge lightens, will accept mating. Ovulation occurs 2-3 days into estrus. Diestrus (60-90 days) — progesterone-dominant, pregnancy or pseudopregnancy. Anestrus (variable) — reproductive rest. Optimal breeding: days 10-14 from first signs of heat, confirmed by progesterone testing. Small breeds may cycle every 4 months; giant breeds every 8-12 months. Spaying eliminates the heat cycle and prevents pyometra and mammary cancer.