Dogs reproduce sexually. Sexual maturity starts around 6 to 12 months for males and females, but it can be delayed for big breeds. Adolescence, which dogs start looking like adults, happens around 12 to 15 months. Dogs are reproductive until old age. A female has its first estrous which means it hits the heat in about 6 to 12 months. This happens once for each 6 months. When this happens, vulva gets bigger and starts bleeding, but after a while, it goes back to normal. The stage is : Proestrus (9 days), Oestrus (9 days), Metoestrus (90 days), and Anoestrus (75 days). A pseudocyesis which is a pregnancy without conception, can happen.
Ejaculation happens in 80 seconds. There are three fractions of semen: clear fluid (Urethral Gland) 0.1 - 3.0 ml, milky fluid (Sperm Rich) 0.5 - 4.0 ml, and clear fluid (Prostate Gland) 1 - 30 ml. Sperm can survive in the virginal tract until the female ovulates. Pregnancy goes for 58 to 68 days. Each litter can have a different dad. A litter consists of 6 to 12 puppies. There is a reproductive disease called canine brucellosis, which sexually transmitted and increases the probability of spontaneous abortion
A bitch may reach sexual maturity anytime from 6 to 12 months of age and this is indicated by the first oestrus cycle, more commonly referred to as "coming on heat" or "coming into season".From this initial season, bitches come on heat every six months (for most bitches), and each oestrus period lasts about 18 – 21 days, but this differs from dog to dog, and it can vary from 14 – 28 days.