By D. Caroline Coile, Ph.D.
Bitches are in heat for about three weeks, but they're typically only receptive to mating for about four to eight days. Many breeders don't allow mating over periods longer than four days, however, for fear puppies conceived later could be born as much as a week premature. In fact, so-called runts are often believed to be the result of later conceptions. But are they really?
No. Despite the fact that the bitch may allow mating for a week or even longer, her eggs are all released together – within at least 48 hours of one another. It takes the eggs several days more to mature. This may occur a week or more after the initial mating – yet even with just one early mating, pregnancies usually occur.
So, how do the sperm cells survive so long? They attach to the wall of the uterine tube and wait. When the eggs mature, their chemical makeup changes. The sperm detect that change, detach from the wall and rush in at once to fertilize the eggs.
So at most, fetuses may differ in age by a day or two – as determined by differences in ovulation – and probably not even by that much. Runts are more likely caused by bad luck in relation to where that particular puppy embryo was implanted, happening to find a place that didn’t receive as much nourishment as others. Once the runt bellies up to the nursing bar, he’ll usually catch up in short order!