It will help you to understand the bitches estrus cycle. As you probably know, female dogs come in heat about twice a year. The heat (or estrus) will last about 3-4 weeks, and in the normal course of breeding, the bitch will stand to be bred somewhere between 10 and 15 days along. This varies a lot, however, and some bitches need to be bred as early as 5-6 days, others as late as 25-30.

The three most important hormones in the bitch's estrous cycle are LH, progesterone, and estrogen.

Estrogen is the hormone that causes the bitch to act receptive. Estrogen causes the vulvar swelling, the bleeding, and the changes in the vagina that many veterinarians use to do "smears". In the average bitch, estrogen rises anywhere from 4 to 20 days before ovulation. Because of this very wide range, estrogen levels are only a very rough estimate of when the bitch will be fertile. Estrogen plays no actual role in causing ovulation.

LH is lutenizing hormone> LH actually causes ovulation. It is normally at very low levels in the blood, then suddenly surges and drops back to near zero again. two days later, ovulation occurs, and then the eggs must mature for two more days before they become capable of being fertilized. the eggs are mature for two to three days, after which they deteriorate and die if they have not been fertilized. Therefore, if you considered the LH peak to be day zero, the eggs can be fertilized four to seven days later.

This is the basis for our decision on when to inseminate with frozen semen. LH testing must be done daily, because the surge only lasts 20-30 hours. This is the most precise way to determine ovulation, so it is the preferred testing choice for frozen breedings. This degree of accuracy is not essential for most other uses (fresh, chilled or natural breedings) so is usually used only on frozen breedings.

Progesterone is also normally at very low levels, then as the heat progresses, progesterone will rise fairly abruptly. It remains elevated for 8-9 weeks (the duration of the pregnancy) whether or not the bitch is pregnant or was bred. At the end of the pregnancy (or non-pregnancy), the progesterone drops abruptly. This sudden drop causes labor to start and is also responsible for the symptoms of false pregnancy, which are often seen in dogs. In fact, all bitches go through false pregnancy with every heat cycle, some just exhibit signs more blatantly than others do.

The way we use progesterone for ovulation timing is to look at the initial rise from baseline levels. This rise occurs within a day or two of the LH surge. There is some variation, but the general rule is that the rise in progesterone from below 2.0 to above 2.0 occurs approximately at the time of the LH surge. Because progesterone continues to rise (versus a quick surge) it can be done every two to three days in most cases. This type of ovulation timing is used for many reasons....fresh chilled breeding, infertility evaluations, natural breeding with limited stud access, and as an adjunct with LH for frozen semen breedings.

As previously stated, the LH surge is the most important event in determining fertile period, breeding or insemination timing, and expected whelp or caesarian section date. Whether you measure LH or estimate the surge based on progesterone levels, the following time frames are important. The day of the LH surge is considered to be day 0, all other times count from there.

+2......Ovulation occurs (immature eggs are released from the ovaries).

+4......Eggs are mature and capable of being fertilized.

+6 to +7......Unfertilized eggs die and are expelled or absorbed by the bitch.

+65......Due date, planned c-section can be done safely on days 63, 64, or 65.

+2 to +6......Insemination times for natural breeding or fresh AI. Optimally, I will pick days 2 and 4 or days 3 and 6. Since we are estimating the LH surge for these types of breedings and fresh semen lives a relatively long time, I tend to error on the side of being a bit early.

+4 and +6......Insemination dates for fresh-chilled breeding. Again, here we are usually using Progesterone to estimate the LH surge, but because of shorter survival of chilled semen, we cannot inseminate too early. Depending on day of the week and availability of overnight shipping we sometimes vary a bit...days 4 and 7 for instance, to accommodate a weekend.

+5 or +6......Insemination dates for frozen semen. Frozen breedings are almost always done surgically, and the timing must be precise. Doing daily LH tests is best for these types of breedings, so your LH surge is as accurate as possible.