Canine Fetal development
In the majority of female dogs parturition occurs between 64-66 days, after the ovulatory surge. Using the date of mating as a reference point as observed in cases of just a single mating or insemination, birth can occur as early as 56 days later and as late as 68 days later.
Part of the explanation is that dog sperm in some cases, survive in the dogs tract for up to 7 or 9 days and still remain viable in terms f being able to achieve fertilization and result in pregnancy. It is possible that in dogs as in other species, many sperm die or loose fertility after 1 or 2 days. However, the number that retains fertility for 2 days is sufficiently high in dogs that mattings do not affect fertility and fecundity on the day of the LH surge, 2 days before ovulation. Thus, in such pregnancies, the sperm survived 2 days before potentially penetrating the oocyte, and the chromatin had to survive another 2 to 3 days to function as a male pronuclear, which fuses with the female pronuclear to form the 1-cell zygote. Fertility declines with mating’s earlier than the day of the LH surge (i.e. mating 3 or more days before ovulation).
The beating fetal heart may be seen as a distinct flickering by day 23-24, but may not be easily detected until day 25. U/S equipment of greater or lesser resolution may detect these and other landmark states or events earlier or later, respectively. Changes in embryo diameter, crown rump length, biparietal head diameter, and other parameters have been well characterized, at least for research dogs. These data have also been successfully applied to estimating gestational age clinically on many breeds based on U/S exam findings, and considering a 0-2 day error depending on the size of the animal and other factors (Kutzler et al, 2003). Relational, non-quantitative changes may be more precise and less breed or animal dependent: e.g., the time when the fetal crown-rump length first exceeds the length of the placental...
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