A biparietal diameter of 7.8cm is about 31 weeks, but this indicator is affected by a number of factors and varies somewhat. The biparietal diameter is one of the necklines of the fetal head, and is the distance between the two parietal bulges. The biparietal diameter of a full-term fetus is around 8.0-10.0 cm, and since there is a close relationship between the size of the fetal head and the number of weeks of gestation, ultrasound instruments can be used to measure the biparietal diameter of the fetal head in order to estimate the number of weeks of gestation and the expected date of delivery. According to the standard curve of biparietal diameter in China, the biparietal diameter of a fetus at 31 weeks is 7.7cm, and at 32 weeks, the biparietal diameter of a fetus is 7.9cm, so the gestational week of a fetus with a biparietal diameter of 7.8cm is about 31 weeks. It should be noted that there is no absolute and strict correspondence between the biparietal diameter and the gestational week, subject to different countries, regions, races and different types of ultrasound equipment and other factors, the measured value varies, the presumption of gestational week is only for reference.