What is the most reliable method to diagnose pregnancy in dogs in late gestation?

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Ultrasonography is the most reliable method in late gestation because it lets you directly visualize the fetuses, assess their viability through fetal heartbeats and movement, and monitor development in real time without exposing the dam or puppies to radiation. This makes it dependable regardless of how enlarged the uterus is or the dog's body condition, unlike palpation which becomes unreliable as gestation progresses. X-ray radiography can show fetal bones and help count pups, but it depends on bone mineralization and involves ionizing radiation, plus it provides limited information about fetal wellbeing. Relaxin testing can confirm pregnancy earlier, but in late gestation its result is less informative and more variable. Overall, ultrasound provides the most consistent, comprehensive view of pregnancy status and fetal health as term approaches.