参考答案：A

解析：Presumptive signs, such as amenorrhea and quickening, are mostly subjective and may be indicative of other conditions or illnesses. Probable signs are objective but nonconclusive indicators--for example, uterine enlargement, Chadwick's sign, a positive pregnancy test, Braxton Hicks contractions, and Hegar's sign. Positive signs and objective indicators such as fetal outline on ultrasound confirm pregnancy.