Dogs are mammals and have sexual reproduction, with internal fertilisation as in humans. During the mating procedure the dogs tie and remain back to back for approx 20 mins or so. This is normal though it does not always happen . The female dog (bitch) has two oestrus cycles per year every 6 months or so. Often called being in season or being in heat, it is the only time that she is fertile around the middle of a season. Pregnancy lasts an average of 63 days.