What is one of the key indicators of high blood pressure during pregnancy?

The key indicator of high blood pressure during pregnancy is the presence of protein in the urine, which is commonly associated with conditions such as preeclampsia. When the kidneys are stressed due to high blood pressure, they may begin to leak protein into the urine, which serves as a clinical marker for complications in pregnancy.

Other options listed, such as increased heart rate, unexplained weight loss, or the absence of protein in the urine, do not serve as direct indicators of high blood pressure during pregnancy. An increased heart rate can result from various factors, but it does not specifically correlate with hypertension. Unexplained weight loss can indicate other health issues, and the absence of protein in the urine would not indicate high blood pressure but rather suggest normal kidney function in that context.