If she was in heat and managed to get with a male dog, it's pretty much a given that she will be pregnant.
Some signs you can look for are weight gain andher nipples will get larger. As she gets closer to whelping she will REALLY show a lot of weight gain, if you gently squeeze her nipples you will be able to express some milk from them, and when she is ready to actually have the puppies,l her body temperature will drop really low (for a dog) and she will refuse food.
The normal gestation period for a dog is 63 days or about 9 weeks. When you take her temperature, her normal body temperature will be about 101.5. When her temperature dips down to 99, the birth is imminent and will occur within 24 hours.
How long ago was she in heat? If it was five weeks ago or less, if you do not want her to have the puppies, you can have her spayed and the vet will remove the uterus and the puppy embryos along with it. If you want her to have the puppies, unless she is an excellent example of her breed and you plan to show and breed her, I would spay her as soon as your vet says he will do it after the puppies are here. The sooner you have her spayed, and the younger she is when she is spayed, the less of a chance she has of developing cancerous mammary tumors when she is older. When you spay her, you also completely remove any chance that she will develop pyometra, which is a life-threatening infection of the uterus. If she does develop pyometra at any point during her life, she will have to be spayed if you have any hopes of saving her. Just some important points to consider.
To add to Ghilly's post................ Check the vulva area on your dog. See if it is softer and a bit swollen. Often they will have a slight straw-colored discharge. She may go off her food for a while.
Was this a planned breeding? Do you know how to properly care for a pregnant dog, handle whelping and possible complications and take care of puppies??
Excellent questions, Margot. Too many people are of the mind that "Mother Nature knows what to do" when it comes to dogs having puppies and cats having kittens. They don't realize everything that is involved to safely bring a litter of anything into the world and quite often are not prepared for the problems they encounter along the way. Too many people have been taught (and erroneously so) for far too long, that dogs and cats are basically self-sufficient when it comes to reproduction.
And to add further to YOUR post, if the only animals that were bred each year were the ones that SHOULD be bred, there would not only not be a pet overpopulation problem, the pets that ARE born would not develop the genetic problems that they do as they get older because the parents would be screened for genetic problems before breeding. Folks don't realize that most of the problems that our pets encounter are passed on genetically, and that many genetic problems don't show up until the animal is 5 years or older. Because of this, instead of attributing the problem to genetics, it is misattributed to the animal simply "getting older". I know, I know, I sound like a broken record. Every time someone posts about breeding their dog I get on my soapbox. But it's important for people to hear facts before they go ahead and allow a litter of puppies to be born, and if even one accidental litter is prevented, then it will be worth every time I have typed it.