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By Dr Linda Simon MVB MRCVS Published July 01, 2026
4 mins
Two German Shepherd dogs on top of mountain
Woman with her cute Border Collie dog on bed at home, closeup
Dog breeding age by breed size
Breed Size First Heat (Approx.) Earliest Sensible Breeding
Small breeds 6 - 12 months After 18 months
Medium breeds 6 - 18 months After 18-24 months
Large breeds 12 - 18 months After 24 months
Giant breeds 12 - 24 months After 24 months. Some dogs may need to wait until 30 months
Mom's dog is lying next to her newborn puppy.
happy mixed breed dog sitting outdoors in summer
Dog heat cycle and breeding timing
Day of cycle (from first bleeding) What's happening Ready to breed?
Days 0-7 Proestrus — vulva swollen, bloody discharge; attracts males but rejects them No
Days 7-10 Early estrus — discharge lightens (salmon/straw); she becomes receptive Yes
Days 10-15 Peak estrus / ovulation — most fertile window; flagging; stands for mating Yes
Days 15-21 Late estrus → diestrus — receptivity ends; cervix closes to sperm No
happy mixed breed dog sitting outdoors in summer
Happy kissing irish setter dogs lying in the grass and licking mouth. Pet love, talking dogs concept.
Two cute beagle dogs getting close to each other
Two dogs on the forest hugging
Mother dog take care puppies with love little dogs