Sexually mature female dogs have an estrus cycle or heat...that period of time when they ovulate, are receptive to male dogs, and can get pregnant...every 4-12 months. The average is every 7 months, but it's quite variable.
This period known as being in heat lasts about 3 weeks, but that too, is just an average. For practical reasons we divide the heat into 3 parts, each part lasting, on average, about 1 week:
Week 1: (Proestrus) The female vulva swells and secretes blood and aromas that drive males into frenzies and on a humid night will attract them from miles around. I'm not sure whether the smell travels so far or whether there's a communication system of jungle drums at work. The female may act moody during this stage to both people and other dogs, swinging from affectionate to "snappy" and seem to like to "tease" the males cruelly: they'll lift their tail to allow a sniff, but will bite the male's head off if he tries to mount.
If a male successfully breeds a female during this time, she is unlikely to get pregnant if you want her to, but could get pregnant if you don't want her to. Does that make sense? Well, it's a joke. But it's somewhat true. The sperm could survive in the vaginal/uterine tract until the female ovulates.
Week 2: (Estrus) During this week, where the female is in true estrus, her vulva will still be swollen, but her vaginal discharge will usually stop having visible blood in it, she will ovulate, and will usually let the male...often any and all males... mount her. And assuming no underlying problems, she will probably become pregnant. Possibly from multiple males; in other words, each puppy in the litter could have a different daddy.
This period, while on average, about a week long, can be much shorter or longer.
Things can go wrong; see below.
Week 3: (diestrus) During the last stage of being in heat, the female has already ovulated and is losing interest in sex, but the male(s) will still be determined. She is not likely to allow mounting and if forced to breed is not as likely to get pregnant.
The Tie: Unlike cat sex, which is a wham bam deal, dogs take their time, and once a male successfully penetrates the female (in an inexperienced male this may take some time or even some guidence from a human) there is a bulbous gland at the base of the penis that swells locking it into the vagina. To get more comfortable, the male will usually do a little dance step over the rump of the female and end up facing away from her...this is called the "tie" and naturally lasts about 3-20 minutes. Both the male and female will often have a very special look on their faces.
If you catch a pair of dogs in a tie, you can cause injury to both the male and female genitals and probably get bitten besides, if you try to pull them apart. Drenching the pair with cold water usually works, but alas, it's probably too late to prevent an unwanted pregnancy.
This is from my old vets website..it has answers to just about any question you have..his website is
www.animalpetdoctor.com