"But... but... I just let her outside for a minute!" "Didn't have your shoes on, did ya?"
There is no telling how many times that little conversation has played out. They knew the dog was in "heat" (estrum), but they underestimated the sex drive, which is a mistake. If the boys don't dig in, the girls may dig out. [It's a powerful force in nature. If Bill Clinton couldn't resist it, how do you expect the dogs to show more self-control than a President?]
Of course, there are definitely folks who just don't understand what's going on in the first place. Maybe you're one of them. If so, stay tuned.
Most female dogs have two estrous cycles yearly. That might make you think their cycles are six months long, but they're not. They have two cycles that last about a month, about six months apart. For ten months out of the year, the average female dog's ovaries are pretty much shut down. As far as female hormones go, they aren't producing any more than a neutered male dog during these quite periods. That's why dogs who have had complete ovario-hysterectomies (complete removal of ovaries and uterus) are not likely to need hormone replacment therapy.
Each cycle consists of three parts: the "coming-in", the "being-in", and the "going-out", also known as pro-estrus, estrus and metestrus. Each part lasts about seven to nine days on average. The female is fertile and receptive to the male only during the middle part or estrus, despite the fact that she is attractive to the male throughout the entire cycle. Boy, is she attractive. They can smell her pheromones (hormone-like scent chemicals) for miles -- literally. You may see a motley crowd of mutts assembling, even if your nearest neighbor is in the next county.
During pro-estrus, the vulva (outside female parts) begins to swell, and a bloody discharge develops. The males are surely getting interested at this point, but the female is far from ready. Unfortunately, many inexperienced owners equate this bloody discharge phase with the entire cycle. They mistakenly assume that once the bleeding stops the cycle is over and it's safe to let the dogs return to their normal living arrangements. Whoops! That's the time when the female is becoming receptive and fertile, so this is the best time to get pregnant.
If day 1 is the first day of bloody discharge ("showing color", as the breeders say), then day 9 is the day to get prospective parents together. If you don't want to raise puppies, this is the time to keep the possible parents separated. Better yet, keep them separated for an additional two weeks.
The whole business is going to take three or four weeks. Maintain constant vigilance. Don't under-estimate the sex drive. Don't let her out unless you've got your shoes on and are ready to defend her honor.