31 weeks may refer to a fetus or a newborn, for 31 weeks fetus and 31 weeks newborn head circumference normal range is different: a. 31 weeks fetus 1. normal situation: 31 weeks pregnancy fetus head circumference in about 28.3cm, dual parietal diameter in about 8.0cm. If the fetal head circumference and biparietal diameter values are within the normal range or the deviation is not large, it indicates that the fetal development is normal; 2. abnormal situation: if the fetal head circumference is small, there may be physiological reasons or pathological reasons, such as genetic factors, developmental malformations, malnutrition, etc. At this time, it is recommended that pregnant women go to the hospital and under the guidance of the doctor for examination as well as appropriate treatment. If the fetal head circumference is large, it suggests that the pregnant woman may have too much nutritional supplementation. At this time, the pregnant woman can carry out blood glucose testing under the guidance of the doctor, and pay attention to diet health in time to prevent gestational diabetes. Second, 31 weeks baby 1, the normal situation: 31 weeks newborn head circumference in about 43cm. The specific calculation is: the average head circumference of newborns is 34cm, increasing by 9cm 6 months after birth and 3cm in the following 6 months, reaching an average head circumference of 46cm at 1 week of age, with a slower growth of about 2cm in the second year, and an average head circumference of 48cm at 2 weeks of age; 2. abnormalities: a small head circumference suggests that there may be brain dysplasia, etc., while an oversized head circumference may suggest the existence of Therefore, paying attention to the size of the head circumference can help in the detection of diseases.