The type of twins and their characteristics】 1. dizygotic twin ① Two eggs are fertilized separately to form a twin pregnancy, called dizygotic twin. Dizygotic twins account for about 70% of twin pregnancies. It is associated with the use of ovulation-promoting drugs, intrauterine transfer of multiple embryos and genetic factors. ③The two eggs are fertilized separately to form two fertilized eggs, and their respective genes are not identical, so the two fetuses formed have differences, such as different or identical blood type and sex, but different fingerprints, appearance, mental type and many other phenotypes. ④There are mostly two placentas, or they can be fused into one, but the blood circulation is independent of each other. ⑤ There are two amniotic cavities on the fetal side of the placenta with two layers of amnion and two layers of chorionic villus in between. 2. Monozygotic twins ① A twin pregnancy formed by the division of a fertilized egg is called monozygotic twin. Monozygotic twins account for about 30% of twin pregnancies. The cause of formation is unknown and is not influenced by race, genetics, age, gestational age, or medical origin. (3) A fertilized egg divides to form two fetuses with the same genes, so the two fetuses have the same sex, blood type and appearance. ④The following four types of fetuses are formed due to the different timing of the division of the fertilized egg during the early developmental stages. (1) Monozygotic twins with double amniotic sacs and double chorionic villi: This type accounts for about 30% of monozygotic twins. (1) The division occurs at the mulberry stage (early blastocyst), which corresponds to 3 days after fertilization. (ii) Two separate fertilized eggs and two amniotic sacs are formed. ③Two amniotic sacs are separated by two layers of chorionic villus and two layers of amnion. (iv) The placenta is two. (2) Monozygotic twins with double amniotic sacs and single chorionic membrane: This type accounts for about 68% of monozygotic twins. (1) The division occurs on the 4th to 8th day after fertilization, when the embryo is in the blastoderm stage, i.e. the trophoblast has differentiated and the amniotic sac has not yet formed. (3) Monoamniotic sac monochorionic twins: This type accounts for 1% to 2% of monozygotic twins. (1) The fertilized egg divides on the 9th to 13th day after fertilization, when the amniotic sac is formed. (2) The two fetuses coexist in one amniotic cavity and share a placenta. (4) Siamese twins: extremely rare. (1) The fertilized egg divides after the 13th day of fertilization, at which time the primordial placenta is formed. The organism cannot divide completely into two, forming different forms of conjoinedtwins. Diagnosis】 1. History and clinical manifestations ①There is a family history of dizygotic twins, and multiple embryos have been transferred with ovulation-promoting drugs or in vitro fertilization before pregnancy. ②Heavy early pregnancy reaction. ③After mid-term pregnancy, weight gain is rapid, abdominal enlargement is obvious, and compression symptoms such as lower limb edema and varicose veins appear early and obvious. ④Late pregnancy is often associated with breathing difficulties and difficulty in moving around. 2. Obstetric examination ① The uterus is larger than the number of weeks of menopause. ②Multiple small limbs or more than 3 fetal poles can be palpated in the abdomen in the middle and late stages of pregnancy; the fetal head is small and disproportionate to the size of the uterus. ③Two fetal heartbeats can be heard in different parts, with or without sound zones between them, or the two fetal heart rates differ by more than 10 beats when auscultated simultaneously for 1 minute. The fetal position in twin pregnancies is mostly longitudinal, with two head positions or one head and one buttock common. B type ultrasonography is helpful for diagnosis and monitoring of twin pregnancies ① Two gestational sacs can be seen in the uterine cavity at 6-7 weeks of gestation and two primitive heart tube beats at 9 weeks of gestation. ②It can screen for fetal structural malformations such as conjoined twins and open neural tube malformation. (③To determine the type of twin fetuses, the diagnosis of dizygotic twins can be confirmed by the inconsistent fetal sex. If the fetal sex is consistent, the estimation is based on the thickness of the interval between the two amniotic sacs. The thickness of the interval >2mm suggests double amniotic sacs and double chorionic twins, and the thickness of the interval <2mm suggests double amniotic sacs and single chorionic twins. ④B-mode ultrasound can also help to determine the fetal position of both fetuses.