The reasons for the sudden appearance of black sweat in late pregnancy may be low blood sugar, low blood pressure, anemia and so on. 1. Hypoglycemia: Pregnant women may have low fasting blood sugar or low dietary intake due to their special physical condition, which leads to dark sweating before the eyes of pregnant women. 2. Low blood pressure: pregnant women due to the dilution of blood or due to the increasing uterus compression of the inferior vena cava, resulting in part of the blood can not return to the normal flow, resulting in pregnant women with low blood pressure or postural low blood pressure, resulting in sweating and darkness in front of the eyes. 3. Anemia: If a pregnant woman is malnourished or has anemia in late pregnancy, anemia may cause dark sweating before her eyes. If a pregnant woman has frequent dark sweating in late pregnancy, she should seek medical treatment.