The thickened vaginal epithelium blocks neutrophil migration to the vaginal lumen. Rarely, a normal bitch will continue to display neutrophils in vaginal cytology during estrus. The value of vaginal cytology is often overestimated in breeding management. But one absolute finding in vaginal cytology is the onset of diestrus. The vaginal cytology changes abruptly and dramatically over 24 to 48 hours from a predominantly cornified pattern to a predominantly noncornified pattern.
Neutrophils are present in large numbers.
The first day of this dramatic cytologic change is referred to as Diestrus Day 1 or D1. This information is valuable because it can be used to predict gestation length. Bitches typically whelp 56 or 57 days after the onset of cytologic diestrus. Vaginoscopy Vaginoscopy is a more accurate diagnostic breeding management tool in bitches than vaginal cytology. Under the influence of estrogen, the vaginal folds become swollen, moist, and pink.