The bitch's estrus cycle is a complex event, and depends on the interaction of several hormones. Progesterone and Luteinizing Hormone (LH) are two of the most important in the determination of ovulation and timing of breeding. Normally, LH is present in the bitch's blood in very small amounts. Just prior to ovulation there is a significant increase in the serum LH value. LH will then return to the baseline level within a 24- hour time period.

It is this surge of LH that triggers ovulation and therefore determines the fertile period of the bitch. The LH surge may occur any time from 3-28 days after the first signs of heat (swelling of and bleeding from the vulva). The average is 8-12 days. Ovulation will occur approximately 2 days after the LH surge. Thee ggs will require an additional 2-3 days to mature so that fertilization may take place. The eggs are viable from 48 to 72 hours after maturity. Therefore the fertile period of the bitch averages 4-7 days or 5-6 days after the LH surge.

Progesterone, another important reproductive hormone in the bitch, is present in low levels before the LH surge and begins to rise at about the time of the surge. Progesterone, unlike LH which returns to baseline levels very quickly, will remain elevated during the the entire 60 days of the post-estrus period dropping just prior to whelping. This extended period of elevated levels of progesterone allows less frequent testing. Baseline levels of progesterone in the bitch are below 1.0 ng/ml. When the progesterone levels rise to 1.0-2.0 ng/ml the LH surge has occurred. The first day the values reach the 1.0-2.0 ng/ml range is counted as day 0. The most fertile breeding days are days 4 and 5 . If there is unlimited breeding availability, then breeding on days 3-5-and 7 would be ideal. If there is limited breeding access, the semen is being shipped , the male has several females to breed, etc. then day 5 would be optimum. For example, on Monday the progesterone value was 1.75 ng/ml. Monday is Day 0, Tuesday Day 1, Wednesday Day 2, Thursday Day 3 ....

Typical Canine Cycle