In recent years, the incidence of twin pregnancies has been increasing year by year, and many of them are in advanced maternal age with difficulty in pregnancy. The first problem to be solved is the determination of ovality and membranous nature of twin pregnancies, the more important of which is the determination of membranous nature. If the diagnosis is not clearly made in early pregnancy, and then referred in the middle and late pregnancy, even the best doctors may not be able to make the diagnosis and treatment. Therefore, both twin patients and obstetricians should give high priority to chorionicity determination in early pregnancy. 1. Ovality: It also becomes syncytial sex. Twin fetuses can be divided into two types according to ovality diagnosis: dizygotic twins and monozygotic twins. The two fertilized eggs form their own separate chorionic cavity, amniotic cavity, yolk sac and placenta (the twin placentas can be fused into one placenta). In contrast, monozygotic twins are twin fetuses formed by the division of a fertilized egg before the trimester, accounting for about 1/3 of twin pregnancies, and the two fetuses share a common placenta, whose chorionicity can be divided into three cases. 2. Membranous: i.e., the form of chorionic villus and amniotic membrane composition of twin fetuses. The majority of dizygotic twins, with the exception of very rare cases, show double chorionic membrane and double amniotic membrane, i.e., the two fetal sacs are separated by two layers of chorionic membrane and two layers of amniotic membrane. In monozygotic twins, depending on the time of division of the fertilized egg during the early developmental stages, the following different membranes are produced: (1) Double chorionic membrane and double amniotic sac: the division of the fertilized egg occurs within 72 hours of fertilization, when the inner cell mass (mulberry embryo) has been formed and the outer cell trophoblast of the blastocyst has not yet been transformed into chorion, which develops into two amniotic membranes and two chorionic membranes, called double chorionic membrane and double amniotic sac. monozygotic twins account for about 20-30% of cases. Although the two fetuses share a common placenta, there is no vascular traffic between the two fetuses, so they do not interfere with each other, and even if intrauterine arrest or death occurs in one fetus, there are no adverse effects on the other fetus. (2) Monochorionic twin amniotic sac twins: The fertilized egg division occurs at the early blastocyst stage from 72 hours after fertilization to day 7, when the chorionic membrane has differentiated and formed, but the amniotic sac has not yet appeared, and the inner cell mass divides in the same blastocyst cavity, so two amniotic cavities, two yolk sacs and one placenta are formed in a single chorionic cavity, and the two sacs are separated by two layers of amniotic membrane. This type of twin fetus accounts for the majority of monozygotic twins (about 70%), and because the two fetuses share a single placental chorionic plate and there are traffic vessel anastomoses on the surface and in the depth of the placenta, there is blood exchange between the two fetuses, which may lead to twin transfusion syndrome, selective fetal growth restriction, and twin reverse arterial perfusion sequences (i.e., absent heart malformation) if there is hemodynamic imbalance or uneven placental segmentation ratio Therefore, once diagnosed, pregnant women with such twin fetuses should pay extra attention to obstetric examination and ultrasound monitoring during pregnancy (every 2 weeks if possible) for early detection of complications and early intervention. (3) Single chorionic villus and single amniotic sac twins: the fertilized egg division occurs after the formation of the amniotic cavity (late blastocyst stage), i.e. 8-13 days after fertilization, and since the amniotic cavity and yolk sac have also been formed, there is only one chorionic villus cavity, amniotic cavity and yolk sac, and one placenta. As you can imagine, two fetuses in this situation develop in one amniotic cavity without any separation between them, so the probability of the two fetuses getting entangled and knotted with each other is very high, and an umbilical cord accident may occur at any time resulting in intrauterine death of both fetuses. Overall, the incidence of MCMA in monozygotic twins is about 1%, and the incidence in the whole population is about 4/100,000. (4) A special case of monoamniotic sac twins – conjoined twins: if the fertilized egg division occurs after the formation of the embryonic disc, i.e. after 13 days after fertilization, it can lead to different degrees and forms of conjoined twins, which can be classified as thoracic conjoined, umbilical conjoined, pelvic conjoined and head conjoined according to the different fusion sites of the two fetuses The fusion of the two fetuses can be classified as thoracic, umbilical, pelvic and head. If the diagnosis is clear in early pregnancy, termination of pregnancy will be recommended in most cases. If a patient with twin fetuses has a teleconsultation, please first ask your obstetrician to inform you of the nature of the chorionic villi in early pregnancy. If it is not clearly written on the ultrasound report, you can contact the doctor who performed the ultrasound on you to pull up the original images for judgment, and if the ultrasound doctor is also unable to make a clear diagnosis, you can ask him to save the original images from your computer on your mobile device and upload them to the internet to see if you can make a judgment.