A typical early symptom of pregnancy is morning sickness, characterized by nausea and vomiting, usually beginning around the 6th week of gestation. This symptom results from hormonal changes, especially increased levels of human chorionic gonadotropin (hCG) and estrogen. Morning sickness helps clinicians identify early pregnancy before more definitive signs such as fetal heart tones or ultrasound confirmation. Other symptoms like leg swelling, back pain, or shortness of breath usually occur later in pregnancy due to physiological changes in blood volume, posture, or respiratory demand. Hence, morning sickness is a common, reliable early clinical feature.

Reference: Williams Obstetrics, Cunningham et al., 25th Edition.