Canine pregnancy lasts 63 days. In the first month of pregnancy, female dogs don't look very different, but as month 1 becomes month 2, things start changing.
Pregnant dogs don't gain significant weight in the first month of pregnancy. About day 35, they starts becoming obviously larger. Ask your vet about the best diet for your pregnant dog. It's important that an expectant dog doesn't become obese, as too much weight can affect delivery.
Between weeks 3 and 4 of pregnancy, your dog's mammary glands begin changing. Teats become more noticeable and pinker; they might appear erect. Around the end of the first month, your might notice slight mucus discharging from the vulva. Some dogs experience bouts of morning sickness or vomiting. This usually lasts just a couple of days during the third and fourth weeks.
Via ultrasound, your vet can tell if puppies are in the womb by day 21. At the end of the month, your vet can perform a manual palpation of your dog's abdomen to feel for embryos. At that stage, the fetuses are roughly the size of walnuts.