If it occurs at 40-50 days, which is close to the time of delivery, and there is no obvious change in the belly, then a prenatal check-up is necessary to see if it is a miscarriage, false pregnancy, or the number of puppies is not large, which is extremely serious. In rare cases, the dog may only be pregnant with one child, and there will be no obvious changes in the belly. Therefore, if you want to know the specific reasons, it is best to go to a pet hospital for a B-ultrasound examination.