Yorkshire terriers are in heat for 2 to 3 weeks, although individual dogs may vary. The beginning of the heat cycle, or estrus, is marked by a swelling of the external vulva. This is followed by vaginal discharge. Estrus has ended when the vulva returns to its normal size and the discharge stops.
Ovulation will usually start once the vaginal discharge takes on a watery consistency. This is the time during which she will be receptive to breeding by a male dog. Since sperm can survive for a week in the reproductive tract, it is possible for a Yorkshire terrier to get pregnant any time during her estrus cycle.