The biparietal diameter is the length of the widest part of the fetal head, between the left and right sides, also known as the large transverse neck of the head. It is often used by doctors to see how the child is developing and to determine if there is cephalopelvic disproportion and if the delivery will be successful. At full term, it should be 9.3cm or more. As a general rule, after the 5th month of pregnancy, it basically corresponds to the month of pregnancy. At 28 weeks of gestation, i.e. 7 months, the fetal biparietal diameter is about 7.0 cm, at 32 weeks of gestation, i.e. 8 months, the fetal biparietal diameter is about 8.0 cm, and so on, after 8 months of gestation, an average weekly growth of about 0.2 cm is normal. In the early stages, when the expected date of delivery cannot be predicted by the last menstrual period, the measurement of the fetal biparietal diameter is often used to predict the date of delivery. After the second trimester, it is often necessary to measure this data when estimating the fetal weight.