The gestation period for dogs is about 63 days. This time frame is measured from the day of ovulation to the day that the dog gives birth. As with humans, the gestation period is divided into three trimesters.
The trimesters are about 21 days each, and, like humans, each trimester comes with its own developments and side effects. Some dogs are affected by morning sickness during the third or fourth week of pregnancy, and pregnant females may also seem more tired. Ultrasounds can be used to see the puppies at three weeks, and the mother's belly is noticeably larger by the end of the second trimester, or around day 40.