No, you can tell when they ovulate, it's just so many vets are off in their readings.
Proestrus is the first stage of heat. The bitch shows a variable amount of vulvar swelling and vaginal discharge. The discharge is most characteristically bloody and serum-like (serosanguinous) but may range in color from milky to frankly bloody. Male dogs are attracted to the bitch but she will not stand for breeding, and often tucks her tail under tightly, sits down, or turns on the male dog. This stage lasts an average of 9 days with a range from 0 to 17 days.
Vaginal cytology specimens contain a population of epithelial cells that undergo a gradual transition from a predominance of non-cornified cells to complete cornification. Red blood cells (RBCs), white blood cells (WBCs), and bacteria are present early and disappear as the bitch nears estrus. Veterinarians can only make estimates from these swabs as to when a dog may come into standing heat.
Progesterone is low until near the end of proestrus, when it may begin to rise slightly. Estrogen is the primary hormone being produced, and is the hormone responsible for the physical and behavioral changes in the bitch, and the changes in the vaginal epithelial cells seen by vaginal cytology. Under the influence of estrogen, the vaginal wall becomes very thick and more cornified cells are collected on the vaginal swab.
Estrus
Estrus is the technical name for standing heat. In this stage, the bitch allows mounting and breeding by the male dog. Her vulva is still swollen but may be softer than in proestrus. Vaginal discharge is still present, and classically attains a straw color at this point, although in many dogs it remains serosanguinous. This stage lasts an average of 9 days with a range from 3 to 21 days.
A bitch is in estrus by cytology when she has 100% cornification with >50% anuclear squames. There are virtually no RBCs, WBCs, or debris present. Veterinarians cannot predict ovulation time prospectively by vaginal cytology alone.
Progesterone rises abruptly early in estrus, and estrogen falls off somewhat. The most important hormonal event taking place at this time is the release of a surge of luteinizing hormone (LH). The significance of this is described later. Ovulation occurs during estrus. Until a bitch ovulates, she cannot conceive. Ovulation is the most important event occurring in the heat cycle.
Diestrus
The bitch enters diestrus when she no longer stands to be bred. This is not an all-or-none phenomenon; some dogs are classified as being in diestrus by vaginal cytology and will still stand to be bred. The vulva will gradually decline in size through this stage. Some dogs continue to exhibit vaginal discharge, which usually is mucoid. Most dogs have no vaginal discharge during this stage. Diestrus lasts an average of 60 days and occurs in every dog, whether they were bred or not, and whether or not they conceive.
As the bitch enters diestrus, there is an abrupt shift back to complete non-cornification and WBCs appear on vaginal cytology specimens. This occurs consistently six days after ovulation.
Progesterone remains high throughout this period. All other hormones are fairly low until the time of whelping.
Anestrus
This is the interval between periods of estrous activity. This is a time of reproductive quiescence, when there is minimal hormonal activity and the dog shows no outward physical changes or unusual behaviors. Anestrus lasts an average of 4.5 to 5 months. At this stage, vaginal cytology specimens contain only a scant number cells, all of which are non-cornified.