In the past 20 years, with the increasing maturity of human assisted reproductive technology and the increase of in vitro fertilization and embryotransfer (IVF-ET) pregnancies, the incidence of multiple pregnancies has increased significantly and their maternal and fetal outcomes have become a concern. This article focuses on the screening methods and prenatal diagnosis of fetal malformations in multiple pregnancies.
The incidence of fetal malformations in twin pregnancies
In the same age group, the incidence of fetal malformations in twin chorionic villus (bivalve) twins is twice as high as in singleton pregnancies, while the incidence of all structural abnormalities in single chorionic villus (monochorionic) twins is twice as high as in bivalve twins. The majority of twin pregnancies have inconsistent types of malformations in both fetuses, and only 15% of twin pregnancies have consistent malformations. Congenital heart disease is the most common structural anomaly in twin pregnancies, and one study found a 9-fold increase in the incidence of congenital heart disease in single chorionic villus (MC) twin (MCT) pregnancies, and a 13- to 14-fold increase in cases of twin-to-twin transfusion syndrome (TTTS); the incidence of congenital heart disease was 15.5% in cases of TTTS treated with laser therapy, and The incidence of congenital heart disease is also significantly higher in twin chorionic villus (DC) pregnancies conceived through assisted reproductive technology; followed by central nervous system anomalies, which occur especially in MC twin pregnancies treated with surgery for TTTS, selective growth restriction (sIUGR), ventricular enlargement, microcephaly, etc., and other anomalies such as facial anomalies, gastrointestinal anomalies, skeletal anomalies Other anomalies such as facial anomalies, gastrointestinal anomalies, skeletal anomalies, and reproductive anomalies are common; and TTTS, sIUGR, and twinreversedarterialperfusion (TRA) are complications specific to MC twins. Therefore, twin pregnancies need to undergo a very detailed examination for fetal structural malformations, including ultrasonography, fetal echocardiography and, if necessary, fetal head magnetic resonance imaging (MRI).
2.Importance and methods to determine the chorionicity of twin fetuses
Determination of chorionicity helps to understand fetal outcome. in a retrospective study by Kristiansen et al [9] in 3621 twin cases, the probability of survival of at least one fetus after 22 weeks was 98.2%, 92.3% and 66.7%, while the probability of both fetuses being lost by 22 weeks was 0.9%, 2.4% and 20.8%, respectively; and the rates of miscarriage, preterm delivery and cesarean section were significantly higher in MCDA and MCMA twin pregnancies than in DCDA twin pregnancies. Complications specific to monochorionic twin pregnancies can lead to a variety of maternal and perinatal complications. Therefore, understanding chorionicity not only helps to enhance monitoring of high-risk pregnancies, but also helps to decide on therapeutic measures to reduce complications, and early diagnosis and close fetal monitoring can reduce morbidity and mortality by up to 8%. The most accurate diagnosis of chorionic vaginal ultrasound is made at 7 to 9 weeks of gestation, while at 10 to 14 weeks the detection rate of abdominal ultrasound can reach 100% according to the typical twin fetal peaks. The sensitivity of the “T” sign is 100% and the specificity is 98.2% for the diagnosis of single chorionic twins.
3. Methods and accuracy of screening and diagnosis of fetal malformations in multiple pregnancies
The rapid development of imaging technology has greatly improved the diagnosis rate of fetal malformation, and the most commonly used imaging methods for fetal malformation examination are ultrasonography, MRI (magnetic resonance imaging), fetal echocardiography and CT (computed tomography).
(1) Application of ultrasound imaging in multiple pregnancy
Ultrasonography is practical, cost-effective, safe and easy to apply, and can be combined with color Doppler, 3D and 4D imaging to greatly improve the detection rate of fetal malformations. The detection rate of fetal structural malformations is affected by the week of gestation, examiner’s skills, instrument resolution and fetal position. The detection rate of structural anomalies such as anencephaly, total forebrain, cerebral bulge, hydrocephalus, umbilical bulge, abdominal wall cleft, visceral translocation, abnormal skeletal development, congenital heart disease, etc. varies from center to center and has a large variation, with an overall anomaly detection rate of 1.6% (0.7%-2.8%) and about 40.8% (12.5%-83.7%) of anomalies can be detected in early pregnancy. Therefore, early pregnancy screening for structural malformations combined with structural screening at 18 to 22 weeks is increasingly accepted by more and more centers. There are no specific rules for the interval between ultrasound examinations in twin pregnancies, and some centers specify that for DC twin pregnancies ultrasound examinations are performed every 1 month after 20 weeks of gestation; and after 28 weeks ultrasound examinations are performed every 2 months. However, the image quality of ultrasound is sometimes affected by maternal abdominal wall hypertrophy, low amniotic fluid, and other factors. Therefore, MRI, CT, and even X-ray have become common adjuncts to examine fetal anomalies in recent years, and fetoscopy is even applied to assist in the diagnosis of fetal body surface anomalies in better centers.
(2) Application of fetal echocardiography in twin pregnancies
Due to limitations such as the small size of the fetal heart and the distance of the fetal heart from the probe, the detection rate of fetal precocious heart disease in the low-risk group is generally 8.5% to 25% through general ultrasound screening. However, in more specialized medical centers, the detection rate of fetal precocious heart disease in multiple pregnancies can be as high as 80% to 90% by fetal echocardiography.
Fetal echocardiography was initially used to rule out structural abnormalities of the fetal heart, but is now widely used to assess fetal heart function. In cases of fetal growth restriction in one of the twins, TTTS, maternal combination of gestational diabetes mellitus, and hypertensive disorders of pregnancy, the risk of combined hemodynamic changes and abnormal cardiac function is significantly higher, and therefore, echocardiography is necessary to assess fetal cardiac function in such fetuses. Echocardiography provides a good understanding of the pathophysiological features of these disorders, predicts pregnancy outcome, and guides and monitors intrauterine treatment.
Of course, the use of fetal echocardiography in the prenatal period has many limitations, such as it can be affected by a small fetal heart, fast fetal heart rate, fetal movement, poor fetal position, maternal abdominal wall hypertrophy, anterior placenta, low amniotic fluid, etc. The detection rate of precordial disease is also affected by the inability to perform interventional studies of the fetal circulation, many of the indicators to detect fetal cardiac function are not well validated, and the normal values of the parameters of cardiac function and their results are not well validated in different centers. The normal values of cardiac function parameters and the reasonable interpretation of the results are still biased, however, this method is still by far the most practical way to assess fetal cardiac structure and function. Currently, the combination of 4D spatiotemporal image correlation (STIC), tissue Doppler imaging (TDI), and M ultrasound can significantly improve the detection rate and accuracy of fetal precordial disease. In fetuses with nuchaltranslucency (NT) thickening, venous ductal flow abnormalities, and tricuspid valve flow abnormalities during early pregnancy, 35%, 28%, and 33% of the fetuses have combined precardiac disease, respectively. Therefore, well-developed centers apply fetal echocardiography in early pregnancy to detect fetal congenital heart disease as early as possible. Combining these three indices, it can detect 50% of precocious heart disease in early pregnancy with about 8% false-positive rate.
(3) Application of MRI in the diagnosis of fetal malformations in multiple pregnancies
Since its application to obstetrics in the 1980s, MRI can be applied to fetal anomalies that are not determined by ultrasound, or fetal anomalies that cannot be detected by ultrasound due to low amniotic fluid, thick abdominal wall, or large fetuses in late pregnancy, because of its irreplaceable advantages, such as higher soft tissue contrast, larger imaging field, no influence of acoustic shadowing by cranial halo, no limitation of amniotic fluid volume and fetal position, and no need of sedation. Initially, MRI was mainly used for the examination of fetal central nervous system abnormalities, but in recent years it has been gradually applied to the examination of respiratory, digestive, urinary system and fetal tumors, which is of great help for further definite diagnosis. MRI is also useful in determining the surgical treatment plan after birth, and Sato et al. found intracranial inflammatory changes in the surviving fetus of a twin MCDA fetus that died in utero at 20 weeks of gestation, which was confirmed by histopathological examination of the fetus after induction of labor. Therefore, in twin pregnancies, MRI is widely used in the evaluation of fetal brain abnormalities after surgical treatment of one of the twin fetuses in MCDA twin pregnancies with intrauterine death, TTTS or sIUGR cases, and plays a very important role in the early detection of fetal central nervous system abnormalities and in the assessment of the long-term prognosis of the fetus after birth.
Of course, MRI examination is limited to some extent by the familiarity of the examiner with the structural features of the fetus at each gestational week and the high cost of this examination. This test is usually performed at 24-40 weeks of gestation.
(4) Application of CT in screening for fetal malformations in twin pregnancies
Werner et al [23] examined 17 normal fetuses and 18 abnormal fetuses (cases of bone dysplasia) with MRI or CT on the same day of 3D ultrasound examination, and then used segmentation and reconstruction techniques to synthesize fetal models, resulting in fetal structural malformations The image features of the resulting model are identical to the appearance of the fetus after induction of labor or birth.
4. Diagnosis and prediction methods of the more common anomalies in twin pregnancies
(1) Ultrasonographic diagnosis and prediction of TTTS
Ultrasonographic diagnosis of TTTS
TTTS is a more common and serious complication of MCDA twin pregnancies, accounting for 10% to 20% of all MCDA twins [24]. Ultrasound is the main tool for the diagnosis of TTTS, and Quintero et al [26] developed a 5-grade classification system, which was widely used once it was introduced and is now the most widely used in China. This grading system has been widely used since its introduction and is currently the most widely used grading method in China, and is now used by the National Institutes of Health (NIH) as an important basis for evaluating the efficacy of laser ablation for TTTS. With the development of fetal echocardiography, it has been found that the fetal heart structure and function have been altered in stage I and II cases, such as ventricular wall hypertrophy, atrioventricular regurgitation, and cardiac enlargement, etc. Therefore, some scholars suggest that the alteration of heart structure should be included in the assessment system of fetal severity of TTTS. predicting the clinical value of the fetus after laser treatment.
Validated methods for predicting TTTS
Quintero grading can only assess the severity of TTTS condition and does not predict the occurrence of the disease. There are several methods to predict the occurrence of TTTS: (1) NT thickening: mainly manifested as NT thickening in the recipient fetus from 11 to 14 weeks of gestation and increased NT difference between the two fetuses, with NT difference >20% or more than 0.6mm in both fetuses, the sensitivity of prediction is 50%-52% The specificity is 80% to 92%. If two fetuses have inconsistent amniotic fluid volume and the difference in head-rump length (aCRL) is >12 mm, the likelihood of future complications is 79% and the survival rate is only 50%; if the NT difference is large along with fetal venous catheter flow and tricuspid valve flow abnormalities, the sensitivity of predicting the occurrence of TTTS is higher. (2) It is also important to check the attachment site of the umbilical cord, such as the umbilical cord sail attachment or marginal attachment may be more prone to TTTS. therefore, in early pregnancy, MC twin fetuses need to be carefully examined for cord insertion characteristics. (3) Differences in amniotic fluid volume or differences in weight between the two fetuses of more than 25% [30]. Any difference in abdominal circumference of more than 10% between 14 and 24 weeks of gestation can predict the occurrence of TTTS to some extent.
Limitations of predicting the occurrence of TTTS
All of the above indicators have limitations in predicting the occurrence of TTTS. For early diagnosis of TTTS, monochorionic pregnancy requires ultrasound examination every 2 weeks from 16 weeks of gestation until 26 weeks of gestation to detect the maximum depth of the amniotic pool, fetal growth and bladder size. If the fetus shows a large variation in amniotic fluid volume, weekly ultrasound review is required; once TTTS is diagnosed, additional umbilical artery flow, venous catheter flow, umbilical vein flow, and, in the case of suspected concurrent twin anemia polycythemia sequence (TAPS), maximum middle cerebral artery velocity (MCA-PSV) must be performed. In case of suspected TTTS, a negative ultrasound is required to measure the length of the cervical canal to predict the risk of preterm delivery in terms of maternal testing.
(2) Diagnosis of twin fetuses with uneven growth
A fetal growth imbalance is diagnosed if the difference in fetal weight between the two fetuses is more than 25%, and the incidence of fetal growth imbalance is 10% in both MC and DC twin pregnancies. The formula is (A-B)×100/A (A is the weight of the larger fetus and B is the weight of the smaller fetus). In general, the larger fetus has normal growth and development, while the smaller fetus shows growth restriction. However, accurate prenatal detection of twin fetal growth imbalance by assessing the difference in fetal weight remains suboptimal due to the presence of measurement error, with an accuracy rate of 23% to 61%. The investigators found that a difference in abdominal circumference between the two fetuses of more than 1.3 to predict twin growth imbalance may be superior to the assessment of weight difference. Of course, assessment of placental function, including amniotic fluid volume and blood flow index, is also very important in suspicion of dystocia.
In DCDA dysplasia (FGR), the conventional wisdom is that it is associated with fetal chromosomal abnormalities, intrauterine infection, and abnormal placental function. The pathophysiology of this condition is generally considered to be due to placental share inequality and the presence of placental vascular anastomotic branches. Prenatal ultrasound is relatively difficult to diagnose uneven placental share, but ultrasound can easily determine the insertion of the umbilical cord of the placenta, and if the sail-shaped placenta or the eccentric attachment of the umbilical cord can be a better predictor of sIUGR.
(3) Detection of sintrauterine fetal demise (sIUFD) in one of the twin fetuses
sIUFD is a common complication of twin pregnancies over 20 weeks, accounting for 6.2% of twin pregnancies. The incidence of sIUFD is 5 times higher in monochorionic twin pregnancies than in dichorionic twin pregnancies. It can lead to intrauterine death of the other fetus, preterm delivery, central nervous system damage and other organ damage, and requires close ultrasound examination, including assessment of fetal growth and development, detection of umbilical blood flow and assessment of amniotic fluid volume. In monochorionic twin pregnancies, additional MCA-PSV testing and fetal cranial MRI at three weeks are required to assess its effect on cranial injury in the surviving fetus. The conventional wisdom is that immediate termination of a single chorionic twin pregnancy with sIUFD is not advisable, as this practice increases the number of complications in the immature infant, and that neurological damage to the other fetus, if any, has already occurred within a short period of time and cannot be avoided even if the pregnancy is terminated immediately; therefore, termination at 34-36 weeks is usually recommended. In DC twin pregnancies with sIUFD, termination at term can be considered if there are no other obstetric complications.
5. incidence of chromosomal abnormalities in multiple pregnancies and prenatal diagnosis
In a study by Sperling et al [37], the incidence of chromosomal abnormalities in twin pregnancies was found to be 0.6%. Among them, 8.0% of twin pregnancies with combined fetal malformations were chromosomal abnormalities, and fetal chromosomal abnormalities can involve one or two fetuses in twin pregnancies, which is an important factor affecting twin development. the results of Glinianaia et al. suggested that the incidence of chromosomal abnormalities in twin pregnancies with combined malformations was about 11.5%, among which 2l-trisomy was the most common, accounting for 4.4%. In dizygotic twin pregnancies with maternal age >31 years, the risk of 2l-trisomy in 1 of the pregnancies was higher than in singleton pregnancies. Overall, the risk of chromosomal abnormalities in one dizygotic pregnancy is at least twice that of a singleton, and the risk of aneuploidy in one fetus is at least twice that of a singleton, although aneuploidy in both fetuses is relatively rare. Monozygotic twins are formed by the division of a single zygote, so both fetuses usually have the same karyotype and the risk of chromosomal abnormalities is similar to that of a single fetus. Monozygotic twins can have the same chromosomal abnormality in both fetuses, but also have different phenotypes. The reason for this may be that chromosomal anomalies are only one of the complex factors involved in the development of fetal malformations. Other factors include intrauterine environmental factors, physicochemical factors, and other factors specific to monozygotic twins, such as uneven syncytial division, intrauterine fetal crowding, and placental vascular anastomosis. Since MCT is usually monozygotic, it is speculated that although monozygotic twins share the same genetic background, the above-mentioned factors specific to each fetus and the differences in the sensitivity of each fetus to the intrauterine environment may be responsible for the appearance of identical karyotype but phenotypic inconsistency in the twins. However, Nieuwint et al. have reported that the phenomenon of monozygotic twins with trisomy 21 in 1 fetus and a normal fetus in the other may be related to chromosomal nondisjunction after monozygotic division. Because of recent reports of monozygotic twins with incomplete karyotypic or genetic concordance, it has been suggested that if ultrasound reveals one or both fetuses with fetal abnormalities or serologic screening abnormalities in twins, both fetuses should be sampled separately, even with MCT. In addition, when short tandem repeat sequence analysis of alleles of twins and their parents is performed, monozygotic twins are diagnosed if all alleles tested are identical in DNA analysis, and dizygotic twins are diagnosed if more than one of the loci has different DNA analysis results.