Timing of Events and Consistent Gestation Length.   

Ovulation occurs approximately 48-60 h (2 days) after the preovulatory LH surge. The LH surge (the central critical reproductive event, Day 0) may occur any time from late proestrus to mid-estrus based on observed behavior changes; on average, it is at 1 day after estrus-behavior onset. Gestation length is mistakenly thought to be quite variable because pregnancy can result in intervals from a single mating to partition that range in the extreme from 55 to 68 days; the intervals counting from the first or last of multiple matins can be even longer or shorter, respectively, i.e. 50 or 70 days. Parturition, however, consistently occurs 64-66 days after the preovulatory LH surge in over 95% of normal pregnancies (Concannon, 2003). The discrepancy is due to several phenomena: oocyte maturation does not occur until 2.5 to 3 days after ovulation; the mature oocyte may remain fertile for up to 5-6 days in the extreme, i.e. until 10-11 days after LH surge; sperm may remain fertile for up to 8 days in the female tract; some bitches may abnormally allow mating 5 days before ovulation, others may abnormally refuse to allow mating until 5 days after ovulation; thus, single matings can be fertile from 5 days before ovulation to as late as 8 days after ovulation (i.e., days –3 to + 10 from LH surge). However, the typical bitch has a limited peak fertile period from Day 0 to Day 5 or 6 after the LH surge, with oocytes apparently dying rapidly 1-2 days after oocyte-maturation and with cervical closure occurring between Day 6 and 9 after the LH surge further reducing fertility even when the oocytes retain their viability beyond that time. In cases where sperm are weak or compromised either naturally or due to methodologies of artificial insemination, the fertile period in an individual instance may be limited to a 1-2 days period. Ideally, breeding management techniques will have estimated the time of the LH surge based on one or more parameters including: timing of Day –1 to +1 decreases in vulval and vaginal-mucosal edema; timing of Day 6-9 metestrus/diestrus change in vaginal cytology; and timing of Day-0 rapid preovulatory rise in serum progesterone as well as any acute change in behavior. Such timing can not only provide for accurate prediction of the date of a Day 64-66 parturition, but also permits establishing preferable times for pregnancy testing, for monitoring of normal fetal developmental changes, and of data collection in the monitoring of pregnancies considered high-risk based on prior pregnancy failures.