Twins pregnancy

　　1. Etiology

　　The occurrence of twin pregnancies is related to race, age, number of pregnancies and deliveries, genetic factors, nutrition, season, and the level of gonadotropin in women's serum. In addition, the application of ovulation-inducing drugs and assisted reproductive technology has also increased the incidence of multiple pregnancies.

　　1. Race and region:The incidence of multiple pregnancies varies greatly between different ethnicities and races. The incidence of twin pregnancies in Nigeria is the highest, with a reported incidence of about 50‰ in the Ibaban region in 1969, followed by the United States at 12‰, England and Wales at 11‰, and Japan at the lowest, with 6.5‰.

　　2. Genetic factors:Epidemiological data show that the frequency of twins in people with a family history of twins is 4 to 7 times higher than that of the general population. Monozygotic twins do not have a familial tendency, while dizygotic twins are related to heredity. Dizygotic twins are inherited through the female line of the mother, with less or no role of the father. Some researchers have conducted a familial study on women who have given birth to twins twice or more, and found that 4.5‰ of these women themselves were one of the twins, 5.5‰ of their sisters had given birth to twins, and 4.5‰ of their brothers' children were twins.

　　3. Age of pregnant women:The incidence of dizygotic twins increases with the age of pregnant women, with a rate of 2.5‰ in women aged 15 to 19, and rising to 11.5‰ in women aged 30 to 40. Age has little effect on the incidence of monozygotic twins. Literature reports that the incidence of monozygotic twins in women under 20 is 3‰, and it only rises to 4.5‰ after the age of 40.

　　4. The number of pregnancies and deliveries of pregnant women:Many scholars believe that women who have given birth three times or more are at a high risk of multiple pregnancies, and the incidence of twins significantly increases for those who have given birth to their fourth child or more. In recent years, due to the implementation of family planning by women in Western developed countries, the number of women with high birth rates has significantly decreased, and the incidence of twins has slightly decreased.

　　5. Nutrition:Animal experiments have proven that increasing nutrition can improve the incidence of twins. In France, the incidence of twins before the Second World War was 7.1‰, while during the war it was 3.7‰.

　　6. Environmental factors:Research shows that continuous sunlight exposure can enhance the thalamus' stimulation of the pituitary gland, causing the level of gonadotropin in women's bodies to rise and increasing the incidence of multiple pregnancies. In some areas of northern Finland, the incidence of multiple pregnancies in July is the highest.

　　7, Serum gonadotropin level:The occurrence of twins is greatly related to the level of serum gonadotropin in pregnant women. In Nigerian women, where the incidence of dizygotic twins is highest, their level of gonadotropin has been found to be higher, while in Japanese women, where the incidence of twins is lowest, their level of gonadotropin is lower.

　　8, Ovulation-inducing drugs:Women with infertility can induce multiple primordial follicles to develop and mature simultaneously with ovulation-inducing drugs, which is prone to lead to multiple pregnancies. According to early pregnancy detection by B-ultrasound, the chance of twins increases by 20%-40% in those using human menopausal gonadotropin (HMG), and by 5%-10% in those using clomiphene citrate (CC).

　　9, In vitro fertilization:Since the study of in vitro fertilization, the incidence of multiple pregnancies has also increased significantly due to the introduction of more than three fertilized eggs into the uterine cavity each time.

　　Second, pathogenesis

　　Generally speaking, the maternal changes in twin pregnancies are more pronounced than in singleton pregnancies. The most important change is that the maternal blood volume increases by 500ml more than in singletons. However, it is interesting to note that the average postpartum blood loss in 25 twin pregnancies is 935ml, which is 500ml more than in singletons. Due to the dramatic increase in blood volume and the development of two fetuses, the need for iron and folic acid increases dramatically, making the mother more prone to anemia. Veille et al. (1985) estimated the cardiac function of twin pregnant women using echocardiography, and compared to singletons, the cardiac output increased, but the end-diastolic ventricular volume remained the same. The increase in cardiac output was related to the increase in heart rate and stroke volume.

　　Another maternal change is that the uterine volume and tension in twin pregnancy increase significantly, with its capacity increasing by 10L or more, and its weight increasing by at least 9kg (20Ib), especially in monozygotic twins, where the amniotic fluid can rapidly increase, leading to acute polyhydramnios. In addition to compressing abdominal organs, it may even cause displacement, possibly elevating the diaphragm, impairing renal function, and Quigley and Cruikshank (1977) reported two cases of twin pregnancy complicated by acute polyhydramnios, leading to azotemia and oliguria.

　　The main impact on the fetus is reflected in weight, where growth restriction and preterm birth result in lighter fetal weight. Compared to singletons, although the weight of twin fetuses is slightly lower than that of singleton fetuses before 28 weeks of gestation, the difference is not significant. After 28 weeks of gestation, the weight difference becomes increasingly significant, and by 34-35 weeks, the separation phenomenon of weight is particularly evident. However, it is noteworthy that at this stage, the combined weight of the two fetuses in twins is often between 4000-5000g.

　　In general, the weight difference between two fetuses is not significant, but in cases of twin-twin transfusion syndrome in monozygotic twins, the weight difference often exceeds 500g. As for dizygotic twins, there can also be significant differences in weight, for example, in a case at Parkland Hospital, a female newborn weighed 2300g, was of appropriate age, and a male newborn weighed 785g. Both survived, and in the subsequent growth process, the latter was always behind the former.

　　1. Preterm birth:Due to the excessive expansion of the uterus in twin pregnancy, the incidence of preterm birth is inevitably increased. As early as 1958, Mckeown reported that the average gestational age of twins is 260 days, with half of the twins weighing less than 2500g. Some preterm births are naturally occurring, and some occur after premature rupture of membranes. The incidence of premature rupture of membranes is higher in monozygotic twins than in dizygotic twins, but the cause is unknown. Since the incidence of malpresentation is high in twins, the incidence of umbilical cord prolapse after membrane rupture is also higher than that in single pregnancy. Preterm birth is the main cause of increased neonatal mortality and morbidity in twin newborns. Recently, Pons (1998) reported that the average gestational age of 842 twin pregnancies was 36.2 weeks, with a preterm birth rate of 45.96% and perinatal mortality rate of 39.2‰. The main cause of death is still preterm birth. Compared to single pregnancy, twin pregnancy itself does not bring greater harm to the fetus than single pregnancy, but the incidence of preterm birth in twin pregnancy is much higher than that in single pregnancy, so it is the main risk factor.

　　2. Anemia:As mentioned earlier, about 40% of twin pregnancies will develop anemia, mainly due to insufficient iron and folic acid reserves to meet the growth needs of two fetuses.

　　3. Pregnancy-induced hypertension (PIH):Pregnancy-induced hypertension (PIH) is one of the main complications of twin pregnancy, with an incidence rate 3 to 5 times higher than that of single pregnancy. It is more common in primiparas, with about 70% developing into PIH before 37 weeks of gestation, compared to only 6% to 8% in single pregnancy. The onset time is also earlier than that of single pregnancy, and the condition is more severe, prone to develop into eclampsia. The incidence of small-for-gestational-age infants also increases, and those with ICP are also prone to develop PIH.

　　4. Polyhydramnios:In twin pregnancy, moderate pregnancy is often accompanied by polyhydramnios, like single pregnancy, but it gradually decreases and finally develops into polyhydramnios in about 12%. Acute polyhydramnios is more common in monozygotic twins and often occurs before the fetus can survive, posing a great threat to the fetus.

　　5. Intrahepatic cholestasis of pregnancy (ICP):ICP is one of the common complications of pregnancy in Chinese pregnant women, and its etiology is related to estrogen. Abnormally high estrogen levels during pregnancy, especially in twin pregnancy due to the presence of two placentas, are more pronounced. The main symptoms are itching, elevated liver enzymes, or accompanied by increased bilirubin, jaundice. The main threat to the fetus is preterm birth and intrauterine asphyxia, which can lead to sudden death.

　　6. Abortion:The abortion rate of twin pregnancy is higher than that of single pregnancy. About 20% of those diagnosed as twin pregnancy by B-ultrasound in early pregnancy will have a spontaneous abortion before 14 weeks of gestation, which is 2 to 3 times higher than that of single pregnancy. Abortion may be related to embryonic malformation, abnormal placental development, placental circulation disorder, and relatively narrow uterine cavity.

　　I. Clinical manifestations:Severe early pregnancy reactions, the increase in uterine size does not match the gestational age, excessive weight gain, frequent fetal movements, in the late pregnancy due to the excessive expansion of the uterus, the feeling of abdominal descent increases, at the same time, the diaphragm rises and compresses the heart and lungs, causing difficulty in breathing. Due to the obstruction of venous return, severe edema and even varicose veins may occur in the lower limbs and perineum.

　　Second, abdominal examination:After the second trimester of pregnancy, the uterus increases in size beyond the corresponding gestational age, and small limbs or two or more fetal poles can be felt in multiple places of the abdomen. After 3 months and 5 months of pregnancy, two fetal hearts can be heard with a Doppler and a fetal stethoscope, respectively.

　　1, Pregnancy period

　　The blood volume of twin pregnant women is greater than that of single胎, and the simultaneous pregnancy of two fetuses requires more protein, iron, folic acid, etc. In addition, the absorption and utilization ability of folic acid is reduced, often leading to iron deficiency anemia and megaloblastic anemia. Twin pregnancy is also prone to complications such as gestational hypertension, polyhydramnios, fetal malformation, placenta previa, placental abruption, postpartum hemorrhage, preterm birth, dystocia, intrauterine growth restriction, intrauterine fetal death, abnormal fetal presentation, etc. The presentation of twin pregnancy is mostly longitudinal, with vertex-vertex or vertex-breech presentations being more common, and other presentations are less common. Due to the dilation of the uterus and high pressure during twin pregnancy, it is easy to develop premature rupture of membranes and preterm birth. The average weight of monozygotic twins is lighter. During twin pregnancy, the placental area is large, sometimes extending to the lower segment of the uterus and the cervical internal os, forming placenta previa and causing antepartum hemorrhage.

　　2, Delivery period

　　There are many abnormal conditions during twin delivery, and the types are as follows:

　　(1) Prolonged labor: Due to the dilation of the uterus and the excessive extension of the muscle fibers, primary uterine contractility is easy to occur, leading to prolonged labor. Sometimes, after the first fetus is delivered, the second fetus's delivery time can also be prolonged due to insufficient uterine contractions.

　　(2) Premature rupture of membranes and umbilical cord prolapse: Due to abnormal twin presentation and polyhydramnios, the intrauterine pressure increases, making premature rupture of membranes and umbilical cord prolapse more likely to occur.

　　(3) Abnormal fetal presentation: Because the fetus is generally small, it often accompanies abnormal fetal presentation. After the first fetus is delivered, the second fetus has a larger range of movement, which is easy to turn into shoulder presentation.

　　(4) Placental abruption: After the first fetus is delivered, the uterine cavity volume suddenly decreases, causing the placental attachment area to also decrease, which becomes the pathological basis for placental abruption. In addition, twin pregnancy often complicates with polyhydramnios. When the amniotic fluid is discharged, the uterine cavity volume decreases, and placental abruption can also occur.

　　(5) Breech presentation and fetal head collision: Clinical cases are rare. If the first fetus is breech and the second fetus is vertex, when the first fetus's head has not been delivered during labor, the second fetus's head has already descended into the pelvic cavity, and the necks of the two fetal heads are locked together, which is called breech presentation and causes dystocia. When both fetal heads are vertex and enter the pelvis simultaneously, they collide with each other, causing obstructive dystocia known as fetal head collision. These situations are more likely to occur in cases where the fetus is small, the pelvis is large, the second fetus has an early rupture of the amniotic sac, or in monochorionic twins.

　　(6) Postpartum hemorrhage and puerperal infection: Due to the excessive stretching of the uterine muscle fibers leading to insufficient uterine contraction, the labor process is prolonged. In addition, the placental attachment area is large, often resulting in postpartum hemorrhage. Because of the multiple complications of twin pregnancy, anemia is common, resistance is poor, and there are two vaginal deliveries during labor, which also makes it easy to develop puerperal infection.

　　1. Strengthen nutrition:The two fetuses require a large amount of nutrition, and if the pregnant woman does not get enough nutrition, it will affect the growth and development of the fetus and the health of the mother. Therefore, the pregnant woman should increase the quantity and quality of nutrition and pay attention to the reasonable allocation of basic nutrients. If the pregnant woman has severe edema, she should increase the intake of protein, and in necessary cases, intravenous infusion of albumin preparations can be given, and a low-salt diet should be adopted.

　　2. Prevention of anemia:The incidence of anemia in twin pregnancy is about 40%, and iron and folic acid should be supplemented regularly. Severe cases should be treated under the guidance of a doctor.

　　3. Prevention of miscarriage and preterm labor:Twin pregnancy has a higher risk of miscarriage than singleton pregnancy due to relative narrow uterine cavity and placental blood circulation disorders, about 2-3 times higher. Therefore, it is necessary to strengthen pregnancy protection and supervision. If one fetus dies, the other fetus can still continue to grow and develop. The dead fetus will be absorbed or squeezed into a paper-like fetus and delivered with the normal fetus, so there is no need to worry or fear, and there is no need to induce labor to terminate pregnancy. Because twin pregnancy causes the uterus to be overly expanded, it is easy to have preterm labor, so it is necessary to pay attention to rest after the second trimester of pregnancy, avoid sexual activity, and make preparations for delivery 4 weeks in advance.

　　Firstly, biochemical tests

　　Since the placenta of twins is larger than that of singletons, in biochemical tests, the average levels of blood chorionic gonadotropin (HCG), human placental lactogen (HPL), alpha-fetoprotein (AFP), estrogen, and alkaline phosphatase are indeed higher than those of singletons, but these methods have no diagnostic value. Only a significant increase in AFP can increase the alertness of people to malformations.

　　Secondly, B-ultrasound examination

　　It is an important tool for diagnosing twins and also has the function of distinguishing the growth and development of the fetus, observing whether the fetus has malformations, and whether there is too much or too little amniotic fluid.

　　1. Diagnosis of twins during early pregnancy:The earliest appearance of pregnancy by abdominal B-ultrasound is at 6 weeks, and general pregnancy can be detected with two fetal sacs in the uterus at 7-8 weeks. However, vaginal ultrasound can detect twins earlier than abdominal B-ultrasound. By 9-13 weeks of pregnancy, the two fetal sacs and their movements are clearly identifiable. After 16 weeks of pregnancy, the biparietal diameter can be measured to observe the growth of the fetus. If a bicornuate uterus is encountered, due to pregnancy in one corner, the decidua in the opposite corner is affected by the ovary and placenta, leading to sufficient development of the decidua, and the secretion of glands fills the cavity, creating a pseudohypertrophic appearance and misdiagnosing it as twins.

　　During early pregnancy, the number of twins diagnosed by B-ultrasound is lower than that of actual twin births during mid-late pregnancy. This is because one of the twins may die for various reasons during early pregnancy, and the incidence of one twin disappearing or dying in the uterus ranges from 20% (Jones et al., 1990) to 50%. This phenomenon is called the vanishing twin, and the rate of multiple pregnancies during early pregnancy in all natural pregnancies is 12%, but only 14% can survive to full term. The risk of miscarriage in monochorionic twins is significantly higher than that in dichorionic twins.

　　2, Diagnosis and monitoring of mid-late twin pregnancy:By the middle and late stages of pregnancy, the accuracy of diagnosing twins by ultrasound reaches 100%, in addition to the appearance of two fetal heads or torsos and visible fetal hearts with different heart rates, attention should be paid to the position of the twin placenta. On the one hand, it is necessary to distinguish between monozygotic and dizygotic twins, and on the other hand, it is necessary to be aware of the possibility of placenta previa or low-lying placenta. In the late stages of pregnancy, the growth rate of the two fetuses in twin pregnancies is slower than that of single pregnancies, and the two fetuses may not be of equal size. If accompanied by twin-twin transfusion syndrome, the differences between the two fetuses are more obvious. Therefore, it is necessary to measure reference values such as biparietal diameter, femur length, abdominal circumference, etc., for the two fetuses to judge the development. In addition, it is necessary to pay attention to the monitoring of amniotic fluid. Joern et al. (2000) used Doppler ultrasound to monitor the umbilical blood flow velocity of the fetus in the late stages of twin pregnancy to judge the prognosis of the fetus. Those with abnormal umbilical blood flow velocity, small for gestational age, preterm birth, cesarean section, and perinatal mortality are significantly higher than those with normal values, so it can also be used as one of the monitoring methods.

　　3, Diagnosis of twin malformations:The incidence of fetal malformations in twin pregnancies is significantly higher than that in single pregnancies. Common malformations include hydrocephalus, anencephaly, meningocele, umbilical hernia, visceral ectopia, biconcave malformation, and heartless malformation, etc., which can all be diagnosed by ultrasound.

　　Three, X-ray diagnosis

　　X-ray examination was once an important method for diagnosing twins, but compared with ultrasound, its diagnosis must be used after bone formation, and maternal obesity, excessive amniotic fluid, and fetal movement can all affect the accuracy of diagnosis, and radiation has certain harmfulness, so it is not as good as ultrasound, which can observe the structure of the fetus from multiple perspectives, measure its diameter, and can be used repeatedly, so it has almost been replaced by ultrasound now.

　　1, It is necessary to strengthen dietary regulation during pregnancy to prevent the occurrence of anemia. Twin pregnancy women require more calories, proteins, minerals, vitamins, and other nutrients to ensure the growth and development of two fetuses. The blood volume of twin pregnancy women is significantly larger than that of single pregnancy women, and the iron requirement is also increased, often resulting in anemia in the early stage. To prevent anemia, in addition to strengthening nutrition and eating fresh lean meat, eggs, milk, fish, animal liver, and vegetables and fruits, iron supplements and folic acid should be appropriately supplemented daily. One can take 1-2 tablets of ferrous sulfate (300-600 milligrams) daily.

　　2, The uterus of a twin pregnancy woman is significantly larger than that of a single pregnancy, and it grows rapidly, especially after 24 weeks. This not only increases the physical burden on the pregnant woman but also, due to the compression on the heart, lungs, and inferior vena cava, it can cause symptoms such as palpitations, difficulty breathing, lower limb edema, and varicose veins, which are more pronounced in the later stages of pregnancy. Therefore, in the later stages of pregnancy, it is especially important to avoid overexertion, rest more, which is beneficial for reducing compression symptoms, increasing uterine blood flow, and preventing preterm labor. Moreover, due to the excessive enlargement of the uterus caused by twins, it is often difficult to maintain until full term and may result in preterm delivery. Therefore, twin pregnancy women need to be hospitalized early for childbirth to ensure a smooth delivery for the mother.

　　3. The main诱发 factors of preterm labor are improper rest and uncontrolled sexual activity. Therefore, pregnant women with twin pregnancies should pay special attention, and should rest more in bed after 28-30 weeks of gestation, preferably in the left lateral position, not in a sitting position, semi-sitting position, or supine position. The left lateral position can increase uterine blood flow and reduce the pressure and dilation on the cervix by the fetus.

　　I. Management during pregnancy

　　1. Nutrition:Supplement iron, avoid overexertion, and rest more in bed after 30 weeks of gestation. It should be ensured that there is enough energy, protein, minerals, vitamins, and fatty acids to meet the needs of the growth and development of two fetuses. In addition to the original 10460J (2500cal) per day, iron should be increased from 30mg to 60-100mg per day, folic acid from 400μg to 1mg per day to prevent anemia, and the restriction of sodium salt may not be beneficial to pregnant women.

　　2. Prevention of the occurrence of pregnancy-induced hypertension:Twin pregnancies can increase the incidence of pregnancy-induced hypertension syndrome, especially in primigravid women. As reported by Hardardottir et al., multiple pregnancies are more prone to upper abdominal pain, hemolysis, and thrombocytopenia, with early onset and severe symptoms, therefore, prevention is very important. Baseline blood pressure and mean arterial pressure should be measured in the early stage of twin pregnancy to facilitate comparison in the middle and late stages of pregnancy. After 24 weeks of gestation, a combination of acetylsalicylic acid (aspirin) 50mg or rhubarb can be taken daily to prevent its occurrence.

　　3. Close monitoring of fetal growth before delivery:During pregnancy, B-ultrasound should be systematically used to monitor the increase in biparietal diameter and abdominal circumference of the two fetuses, while also paying attention to the inconsistency in the growth of the two fetuses. If the abdominal circumference of the two fetuses differs by 20mm or more, the weight will differ by 20% or more. If they are of the same sex, the possibility of TTTs should be considered; the greater the weight difference, the proportionally higher the perinatal mortality rate will be. Additionally, Doppler ultrasound can be used to measure the difference in blood flow velocity in the umbilical vein and artery of twin fetuses to distinguish the inconsistency in their growth. The amniotic fluid volume of twin pregnancies should also be paid attention to.

　　4. Prevention of preterm labor:The use of beta-adrenergic receptor agonists can prolong the duration of twin pregnancy and increase fetal weight. Cervical insufficiency can undergo cervical cerclage under the premise of using uterine contraction inhibitors.

　　5. Actively treat complications of pregnancy.

　　6. Actively treat intrauterine growth restriction and twin-to-twin transfusion syndrome.

　　II. Management during the delivery period

　　1. Selection of delivery mode:The management of twin deliveries first involves selecting the mode of delivery, which should be determined based on the health condition of the pregnant woman, her past delivery history, the current gestational age, the size of the fetus, the position of the fetus, and whether she has any complications and what kind of complications. Twin deliveries are different from singleton deliveries, as twin pregnancies have more complications, a longer labor, and more postpartum hemorrhage, all of which are factors that must be considered. The goal is to ensure the safety of the mother, and to strive to reduce the perinatal mortality rate, with fetal weight and position often being the most important determining factors.

　　2. Cesarean section:Currently, there is an increasing trend in choosing cesarean section as the mode of delivery in twin deliveries. Chervenak (1985) reported a cesarean section rate of 35%, Parkland Hospital reported nearly 50% in 1993, and 53% in 1994. The main indications for surgery are nonvertex, followed by uterine atony, hypertensive disorder complicating pregnancy, and fetal distress. Relaxing the indications for cesarean section can reduce the perinatal mortality rate of twins, and the perinatal mortality rate of cesarean section twins is 0.89%~1.45%, while the mortality rate of vaginal delivery is 6.23%. The indications are:

　　(1) Hypertensive disorder complicating pregnancy and severe placental dysfunction.

　　(2) The weight difference between the two fetuses is greater than 20%.

　　(3) Incomplete fetal and cervical maturity, resulting in uncontrollable uterine contractions.

　　(4) Abnormal fetal position: Twin pregnancy can be seen in the following fetal positions, such as one head and one breech (26%), two vertex (40%), one breech and one head (10%), two breech (10%), one head and one transverse (8%), one breech and one transverse (2%), other abnormal positions (4%). In addition to the first two, other fetal positions are recommended for cesarean section.

　　(5) Other indications for cesarean section are similar to singletons. If the gestational age of the fetus is 34 weeks or more, or the weight is above 2000g, the position of the fetus is the main factor determining the mode of delivery. If both are vertex, or the first baby is vertex, cesarean section can be considered; if the first baby is breech or other positions, cesarean section is advisable, because when the first fetus is delivered, if the second fetus is delivered vaginally, whether it is an internal version or breech delivery, the neonatal mortality rate is 6 times higher than that of vertex. In recent years, when the second fetus is breech, after the first fetus is delivered, B-ultrasound can be used to find the fetus's feet, which is conducive to breech extraction surgery, and the mortality rate is close to cesarean section. However, the problem still lies in the fact that if the surgeon does not have experience in internal version or breech extraction, cesarean section is still advisable.