Let’s go back to the beginning—your Whippet bitch is pregnant—how did she get that way when a certain percentage of attempts to mate Whippets fail?
Whippet bitches have a much higher degree of normal variation in the length and timing of their cycles, and the dates of ovulation, than do most other breeds of dog. A perfectly healthy and normal Whippet bitch may experience first heat at any age between 9 months (uncommon) and 3 + years (also uncommon), and within this range the majority of fertile and productive individuals have been found. In the author's experience, bitches are unlikely to cycle prior to their first birthday. Those who experience first heat between the ages of 12 months and 24 months are generally going to be biannual cyclers, more or less, with periods between heats of 6-9 months. Those who experience first heat (estrus) between the ages of 2 years and 3 + years are typically closer to the annual cycling model, with heats spaced between 11 and 13 months apart. I imagine this is part of the mixed heritage of the breed. Some cycle more like the terrier and Italian Greyhound portion of the background gene pool, while others cycle in a manner more similar to the large open field coursing hounds like the greyhound. To cycle biannually is not a disadvantage to the toy and terrier, while to cycle annually gives an extended period of the year during which the female is able to course with the coursing pack and remain at peak fitness. I have no evidence, but I believe this explains the variation we see in our breed. Both patterns are NORMAL and independent of fertility (how many puppies can be conceived and carried to term).
Those individuals who are later cycling and have a more annual period between heats are more likely to be what I call "late acceptors". For these bitches, accepting the male for the first time (that is, when the bitch will stand willingly without attempting to turn around and take the poor male's head off) is going to occur much later than the 8-11 days post first discharge seen that is commonly cited in the breeding manuals as a good time to attempt a mating. If subjected to progesterone testing, these bitches are generally shown to ovulate around day 14-22 post first sign of colored discharge. In these cases, it is generally best to trust to nature. I have had a litter of 8 born of a breeding on Day 25, following a forced mating seven days earlier (by forced, I mean that the bitch was "hogtied" and restrained so that the male could tie her without suffering injury). But on Day 25, she screamed to be with the male and crawled under him, willing him to breed her one last time, and based on the delivery date, and the fact that the pups were of average size and their amniotic fluids and placentas were not of the color and consistency that would indicate she had carried them past term, I presume this was the productive mating. I have had another bitch who conceived a litter of 8 on Day 25 when she had been quite without discharge for several days and I assumed she was essentially out of season. But a lapse in my attention allowed the male access, and she stood and he tied her easily, and fortunately this was a pedigree I would have done in any event—a planned breeding at an unplanned time. So, very late acceptors are NOT uncommon. And given that the eggs are ripe for fertilization for only a short period of time following ovulation, breeding a week too early or even a minute too late is the reason for most of the litters that “miss”.
Because of this, I tend to trust nature, and my bitch, and am no longer so quick to "force" a breeding on an obviously unwilling bitch, no matter how much the male may be willing to give it a go. The second complicating factor is that Whippet bitches are highly individual in how long they are willing to stand for a male in order to be bred. Some are true "slut puppies" and will allow a male to mount them even long after they are out of season. Others have a very brief window during which they will allow a breeding to take place. I have had bitches who had a window of less than 24 hours in which they would let the male anywhere near them, and others that didn't even mind being mounted--or even tied!—once they were well out of season (day 40!!). Both types of bitches were fertile and able to conceive litters of 7 or more. I would encourage stud dog owners, for this reason, to put their sires with bitches sent to them twice daily, both morning and night, until at least one mating is accomplished. There are bitches who will continue to accept a male long past their fertile period, and others who, once a productive mating has taken place, seem to "lock the door and throw away the key" and want nothing more to do with the male. Both types are capable of having a litter of above-average size. Any enthusiastic mating by a Whippet stud with a bitch who stands and flags willingly should be considered a "good" breeding. There is no need with some of our bitches for there to be more than one mating to ensure that the bitch has been "caught" in time. My personal preference is for two days back to back, and then another breeding two to three days later if possible. The fact that Whippet bitches can have long or delayed acceptance periods can be tricky for those who are attempting to drive their bitch to the stud dog and back at the time during which they believe the mating will be productive.