The best biparietal diameter for a normal delivery is not talked about as the best, it has to be compatible with your week of pregnancy. It also depends on the height and pelvic size of each woman. Each week of pregnancy has a corresponding average value of the biparietal diameter, as long as it is compatible with the average value of their own week of pregnancy, for example, the average value of the biparietal diameter of 37 weeks is about 9.0cm, if 38 weeks is 9.08cm, 39 weeks is 9.21cm, 40 weeks is 9.28cm, which are talking about centimeters, talking about the length of the biparietal diameter. All values throughout pregnancy have an average number, and the biparietal diameter is no exception. So it’s best to be around this average, for example 40 weeks don’t be 37 weeks biparietal diameter, it’s on the small side. On the other hand, a 37-week pregnancy should not be a 40-week biparietal, and that would be on the large side. It’s not optimal to talk about, but it’s generally compatible with a smooth vaginal delivery, provided of course that the pregnant woman’s pelvic conditions are normal, such as a particularly narrow pelvis is also not possible.