Practical Matters: Vaginal cytology for ovulation timing: Not necessarily an accurate indicator
Veterinarians cannot identify an ovulation date or a bitch's fertility from vaginal cytology. All that can be identified is whether the bitch has been exposed to estradiol and at which stage of the cycle she is in at the time of sample collection. The average bitch ovulates two days after the onset of cytologic estrus, defined as complete cornification with more than 50% of the cells appearing anuclear. However, many normal bitches are not average, which makes vaginal cytology results too inaccurate to consistently determine the ovulation date. The ovulation date can be most accurately identified prospectively by measuring luteinizing hormone or progesterone concentrations in blood.
Margaret V. Root Kustritz, DVM, PhD, DACT