The mature human egg cell has a diameter of .12mm, which is a small white spot visible to the naked eye and round under the microscope. The length of sperm is .6 mm. Under the microscope, the head of X sperm is relatively large and round. The head of Y sperm is smaller and sharper. X and Y sperm have different shapes because they carry different DNA. Eggs and sperm are fertilized in the fallopian tube and begin to grow after fertilization. Different DNA will definitely have different shapes. In the early stage of fertilized egg development, the shape of the egg is basically maintained, which is round. The fertilized egg can grow to 2 mm in the fallopian tube, and it can generally grow to 5 mm when it is successfully implanted. The embryo at this time, dragging a tail, was surrounded in gestational sac. After the fertilized egg is implanted, it is called an embryo. Before the embryo develops its initial brain and spinal cord, that is, before it grows to 13 mm, it basically remains round. Relatively speaking, the maximum diameter is between 13 and 18 mm, and the shape of gestational sac is easy to judge: the gestational sac of XY chromosome is long; The gestational sac of XX chromosome is relatively round in shape. However, the embryos of XX and XY chromosomes are basically round before 12 mm (before the embryonic organ differentiation, they are close to the shape of an egg). After the maximum diameter exceeds 19 mm, the shape varies widely (influenced by the implantation environment of the uterus, for example, if the implantation is good and the villi absorb enough nutrients, the fetal sac will be round; On the contrary, it is slender). Only 13 ~ 18 mm, the embryo has just completed implantation and embryo bud has just begun to form. The shape of fetal sac is mainly influenced by DNA, but less by implantation environment. At this time, sex can be judged according to the shape. It should be noted that the shape of fetal sac is long and round from the anatomical point of view; If it is B-ultrasound, there will be errors. I have done experiments. I put a meatball bought in a supermarket in the abdominal cavity of a cut chicken, and then observed it with B-ultrasound. Due to the different positions of the B-ultrasound probe, the shape of the meatball is round, oblate or even curved. Relatively speaking, the shape of fetal sac obtained by transvaginal B-ultrasound is closer to reality. Therefore, if we just see that the longest diameter of the fetal sac is between 13 and 18 mm through B-ultrasound, we can judge the sex according to the shape: long for a male and round for a female. There are three conditions for this method to infer: 1. It must be Yin Chao and provide three data. If it is abdominal distension, it will reduce the accuracy. 2. Among the three data, the longest diameter is 13 ~ 18mm. This size is not absolute, but the greater the size distance from this interval, the lower the accuracy of judgment. 3, 3 data, two data are basically equal, and the other data is half of the first two data, so it must be a boy; If three data are close to arithmetic progression, or the other data is .7 ~ 2.6 times of the first two approximate data, it must be a girl.