The number of litters per lactation in mammals is usually the number of nipples-1, and in a few cases it can be the same as or even more than the number of nipples. In primates, including humans, the number of nipples has been reduced to 2. Therefore, the number of fetuses is usually single, and in a few cases, it is double or even multiple. The common people see the joy of having two babies, while doctors see the sorrow of having two babies. A pregnancy with two or more fetuses at the same time is called “multiple pregnancy”, of which twin fetuses are the most common. In recent years, with the development of assisted reproductive technology (including in vitro fertilization), the incidence of multiple pregnancies has increased significantly. Multiple pregnancies have the potential to cause many complications for both the pregnant woman and the fetus, resulting in increased neonatal mortality. Therefore, multiple pregnancies, with one side of joy and the other side of worry, are high-risk pregnancies. Some twins are so much alike in appearance, shape, language and personality that it is difficult for ordinary people to tell them apart. A story that the husband’s twin brother to the home as a guest, the brother into the kitchen to help wash dishes, the kitchen left two people when the wife said: “Husband, a long time no that, or not at night …… “The man turned around and said:” I’m sorry sibling, I’m your brother.” The embarrassment is extreme! Finished eating when cleaning up the dishes, the wife said: “Just now I took our brother as you, embarrassed to death!” The man turned around and said, “Sorry sibling, I’m still your brother!” In addition to the story letting everyone know that the brother is very decent, you can also tell that the twins are monozygotic. That is, it was formed by the division of a single fertilized egg. The two individuals have the same genes, the same gender, blood type, appearance, fingerprints, personality traits, etc. Monozygotic twins account for about 30% of twin pregnancies, and the reason for their occurrence is unclear. More often than not, the case is dizygotic twins. Two eggs are fertilized separately to form, their respective genes are not identical, so the two fetuses are different, blood type and sex can be the same or different (a boy and a girl, i.e., Dragon-Phoenix Fetus), and fingerprints, appearance, mental type and other phenotypes are different, genetically the two have the same relationship as ordinary siblings. This type of twin fetus is difficult for the general public to know that they are twins unless the person concerned specifically emphasizes it, and accounts for about 70% of twin pregnancies. There is a clear family history of dizygotic twins, and if a woman is herself one of the dizygotic twins, she is more likely to give birth to a twin baby than if her husband is one of the dizygotic twins, which suggests that the woman’s genetic influence is greater than the man’s as far as the ability to carry a twin baby is concerned. In other words, if two children are conceived, it mostly means that the woman is strong. Certain ovulation stimulating drugs used in infertility treatments can increase the incidence of twin pregnancies. In IVF (in vitro fertilization-embryo transfer), 2 to 3 embryos are usually grown to ensure a successful transfer, which also contributes somewhat to the increased incidence of twin pregnancies. There is also a less common type of twin pregnancy called “simultaneous repeat pregnancy”. The website of The Guardian in the United Kingdom reported a legendary experience shared by a Danish woman, who gave birth to twin boys in 2005, and surprisingly, their biological fathers were men with different skin colors! Some users commented that this was a purely spoofed apocryphal story. In fact, this is not the case, this is the phenomenon of contemporaneous duplicate pregnancy, is two eggs in a short period of time at different times fertilized and formed by the twin eggs, specifically refers to the sperm originated from different males. Simultaneous twin pregnancies are more common in cats, where there are black, white and flowers in a litter, but they are rare in humans, probably due to physiological differences and more to do with moral constraints. In terms of risks to the pregnant woman, there is no significant difference between these types of twin births, all of which can cause more danger than a single birth. Examples include hypertensive disorders of pregnancy, intrahepatic cholestasis of pregnancy, anemia, excess amniotic fluid, placental abruption, premature rupture of membranes, weak contractions, and postpartum hemorrhage, as well as preterm labor and abnormal fetal development. In multiple pregnancies, the early pregnancy reaction of the pregnant woman is heavier and lasts for a long time. After the 10th week of pregnancy, the size of the uterus is significantly larger than that of a single pregnancy, and grows more rapidly after the 24th week of pregnancy. In late pregnancy, due to the excessive enlargement of the uterus pushing the diaphragm upward, so that the lungs are compressed and the diaphragm activity amplitude is reduced, often with breathing difficulties; due to the excessive enlargement of the uterus compression of the inferior vena cava and pelvic cavity, hindering venous return, resulting in lower limbs and abdominal wall edema, lower limbs and vulvar, vaginal varicose veins. For the fetus, or from the doctor’s point of view, monozygotic twins are at greater risk than dizygotic twins because of a condition known as “Twin Fetal Transfusion Syndrome”, which occurs in monozygotic twins. Due to the timing of the division of the fertilized egg in the early stages of development, monozygotic twins can be divided into four different types, one of which is at risk of developing a complication known as “Twin Fetal Transfusion Syndrome”. One fetus becomes a blood donor and the other becomes a recipient, resulting in anemia and decreased blood volume in the donor, leading to growth restriction, insufficient renal perfusion, low amniotic fluid, and even death due to malnutrition; however, the recipient is not without risk, with increased blood volume, increased arterial pressure, and increased size of the organs, which can lead to heart failure, fetal edema, and excess amniotic fluid. In the past, the diagnosis was usually made by postnatal examination of the newborn, and if the difference in weight between the two fetuses was ≥20%, and the difference in hemoglobin was >50g/L, it was suggestive of twin-fetal transfusion syndrome. Currently, the diagnosis is usually made by ultrasound. Ultrasound is currently the primary method of confirming the diagnosis of multiple pregnancies. As early as 6 weeks of gestation (42 days after the last menstrual period), ultrasound can show the number of embryonic sacs deposited in different parts of the uterus, each of which, together with the surrounding meconium, forms a liquid halo characterized by a double ring. After the end of the 7th week of pregnancy, a primitive heart tube with rhythmic beats appears in each blastocyst. After 12 weeks of gestation, the fetal head is visualized and the biparietal diameter of each head can be measured. With the increase of gestational weeks, the correct diagnosis rate can reach 100%. Therefore, clinical suspicion of multiple pregnancy should be followed until the number of fetuses is fully determined. After 12 weeks of gestation, fetal heart sounds of different frequencies can be heard by Doppler fetal cardiography. Pregnant women with multiple pregnancies need to pay attention to the following issues: Supplementation of adequate nutrition: to increase the intake of calories, proteins, minerals, vitamins and essential fatty acids as a principle, appropriate supplementation of iron and folic acid to prevent anemia. Preventing preterm labor: it is the focus of prenatal monitoring of twin pregnancies. Preterm labor is significantly more likely to occur in twin pregnancies than in single pregnancies. If preterm miscarriage occurs before the 34th week of pregnancy, it is necessary to give contraction inhibitors and hospitalization for observation if necessary. Timely prevention and treatment of complications in pregnancy: including hypertensive disorders in pregnancy, cholestasis in pregnancy, etc. Monitor fetal growth and development and changes in fetal position.