Which sign of pregnancy is most accurately associated with a medical diagnosis?

The most accurate sign of pregnancy associated with a medical diagnosis is a positive pregnancy test. A positive pregnancy test indicates the presence of human chorionic gonadotropin (hCG) in the urine or blood, which is produced shortly after a fertilized egg implants in the uterus. This hormonal marker is a direct biological indicator of pregnancy, making it the definitive sign for medical diagnosis.

In contrast, a missed period, while often a common early indicator of pregnancy, can also occur due to other factors such as stress, hormonal imbalances, or other health issues, and thus is not specific to pregnancy. Breast tenderness is another symptom frequently experienced during pregnancy but can similarly result from premenstrual syndrome or hormonal changes unrelated to pregnancy. Uterine enlargement is typically observed later in pregnancy and, while it can support the diagnosis, it is not as immediate or definitive as a positive pregnancy test. Hence, the positive pregnancy test stands as the most reliable sign for confirming pregnancy in a medical context.