Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

438 •DOG OWNER’S HOME VETERINARY HANDBOOK
Estrus
The second phase of the heat cycle is called estrus, or standing heat. This is
when the female is willing to breed. Estrus lasts seven to nine days (the range
is 2 to 20 days). It ends when the bitch refuses to stand for the stud.
As estrus begins, the vulva softens and becomes pliable in preparation for
intromission. The discharge becomes watermelon colored or pink. At this
time the female begins to flirt with the male, raises her tail and flags it to the
side, lifts her pelvis, and presents her vulva when touched in the rear.
Ovulation usually occurs on the second day of estrus, or about the 12th day
of the heat cycle as measured from the first day of proestrus. Keep in mind that
ovulation may occur sooner or later than expected, owing to variations in the lengths
of proestrus and estrus.
A microscopic examination of vaginal secretions (a test called vaginal
cytology) helps pinpoint the beginning of estrus. There are characteristic
changes in the appearance of vaginal cells that tell your veterinarian when
the bitch is in estrus, and later, when she is in diestrus.
Vaginal cytology, however, is not an accurate method of predicting when
ovulation will take place. A more accurate method is to measure serum progesterone
levels. The serum progesterone level remains low during early
proestrus (less than 2 nanograms per milliliter). As ovulation approaches, the
serum progesterone begins to rise. The reason progesterone rises is that it is
stimulated to do so by the rise in luteinizing hormone (LH), as shown on the
chart on this page. Note that LH peaks rapidly and drops precipitously. The
LH surge triggers ovulation, which occurs two days after the peak. As LH
peaks, the serum progesterone rises above 2 ng/ml. Thus, when the progesterone
measures above 2 ng/ml, one can predict that ovulation will occur
within the next 24 to 48 hours.