• Allison SO, Andreotti RF, Lee SI, Angtuaco TL, Horrow MM, Javitt MC, Lev-Toaff AS, Podrasky AE, Scoutt LM, Zelop C, Expert Panel on Women's Imaging. ACR Appropriateness Criteria® multiple gestations. [online publication]. Reston (VA): American College of Radiology (ACR); 2008. 6 p. [46 references]

This is the current release of the guideline.

This guideline updates a previous version: Fleischer AC, Andreotti RF, Bohm-Velez M, Horrow MM, Hricak H, Javitt MC, Thurmond A, Zelop C, Expert Panel on Women's Imaging. Multiple gestations. [online publication]. Reston (VA): American College of Radiology (ACR); 2005. 10 p.

The appropriateness criteria are reviewed annually and updated by the panels as needed, depending on introduction of new and highly significant scientific evidence.

ACR Appropriateness Criteria®

Clinical Condition: Multiple Gestations

Variant 1: Patients with high index of suspicion for multiple gestations—assisted reproductive techniques, large dates for pregnancy, and elevated maternal serum alphafetoprotein.

Variant 2: Patients with low index of suspicion for multiple gestations—all pregnancies or family history of twins.

Variant 3: Initial ultrasound has diagnosed twins on the same scan.

Variant 4: Parameters to measure for twin discordance.

Summary of Literature Review

All multiple gestations are high-risk compared with singleton pregnancies. Dichorionic twin pregnancies, all of which must also be diamniotic, are the safest form of twinning and carry a 10% risk that one or both fetuses will not survive beyond the neonatal period. When twins share one placenta--monochorionic-diamniotic twinning--that risk increases to 25%, with the increased mortality due to complications related to blood vessel communications between the cardiovascular circulations of the individual twins. These conditions include twin-twin transfusion syndrome (TTTS), twin embolization syndrome, and acardius, or twin-reversed arterial perfusion sequence. When twins also share the same compartment--monochorionic-monoamniotic twinning--the loss rate jumps to 50%, with the incremental mortality attributable to cord entanglement accidents.

The major sources of morbidity and mortality common to all twin gestations are prematurity and intrauterine growth restriction, which may affect one or both fetuses. There may be an earlier onset to placental postmaturity complications. There is also an increased incidence of congenital anomalies among all twins, although anatomic malformations occur four to five times more frequently in monozygotic as in dizygotic twins. The overall risk for at least one of a monochorionic/monoamniotic twin pair having a structural congenital cardiac anomaly is eight times that of a monochorionic/diamniotic twin pair. In addition, if one of a pair of monochorionic twins is affected, the risk to the other twin for a cardiac anomaly is also higher. All categories of perinatal morbidity and mortality among twins occur with even greater frequency in higher-order multiple gestations.

It has been the task of the guideline committee to determine, by evaluation of the medical literature and use of consensus techniques, the timing and frequency for assessing the health status of multiple gestations. To date, in spite of the increasing incidence of multifetal pregnancies related to assisted reproduction, there is lack of medical literature regarding imaging schedules for diagnosis and follow-up in this population. The appropriateness criteria presented here have been revised to address diagnosing a multiple gestation in the first trimester, with a scan for detailed anatomic evaluation and comparative growth at 18-20 weeks. Triplet and higher-order multiple gestations are not specifically addressed, but these should all be treated as very high-risk pregnancies.

Growth scans should be performed at least every 3-4 weeks. Some form of fetal monitoring, probably best accomplished by some variant of the sonographic biophysical profile (since it is very difficult to confirm that nonstress tests have successfully interrogated each fetus of a triplet or greater multifetal pregnancy), should be considered on a weekly or more frequent basis once the pregnancy has reached the point of potential postnatal viability. Even closer surveillance may be indicated if there is a monochorionic or monoamniotic twin pair as part of the multifetal pregnancy, particularly if there is discordance in fetal sizes or amniotic fluid volumes.

Initial Ultrasound Exam

Determination of multiple gestation in the first trimester is important for several reasons. Monochorionic twins and triplets are at a higher risk for twin-twin transfusion, fetal growth restriction, congenital anomalies, vasa previa, velamentous insertion of the umbilical cord, and fetal death. Because of the increased morbidity and mortality directly related to problems resulting from chorionicity, establishment of placentation, amnionicity, and chorionicity is essential in implementing an antepartum management strategy. Because of the high risk of perinatal loss, multifetal pregnancy reduction is often offered to reduce number of fetuses with the hope of optimizing the outcome for the remaining fetuses. Chorionicity must be determined prior to reduction because the presence of vascular anastomoses in monochorionic gestations increases the risk of injury or damage to the remaining fetuses.

The first trimester is considered the most optimal time to diagnose chorionicity with ultrasound (US). Sonographic assignment of monochorionicity during the first trimester had higher sensitivity, specificity, and positive and negative predictive values than assignment during the second trimester. As with twins, the techniques of sonographic determination of chorionicity can be applied to other multiple gestations with a high degree of accuracy.

Because first-trimester US for pregnancy is not universally routine, the early diagnosis of multiple gestation via US relies on maintaining a high index of suspicion in patients with uterine size greater than expected for dates, a history of assisted reproduction, and elevation of maternal serum alpha-fetoprotein. Detection rates for multiple gestation in the first trimester approach 99.3%. In the RADIUS trial, 129 multifetal pregnancies were studied. Those screened with US were consistently diagnosed earlier than those not screened. In spite of this, no statistically significant fetal benefits were demonstrated in those who were screened in this trial.

During the initial scan it is also important to assess the size of each twin and the degree of discordance, if any, between them; to evaluate the amount of amniotic fluid for each; and to image the cervix to check for changes of effacement or dilatation.

Follow-up Ultrasound Exam/Antepartum Surveillance

Tests for evaluating fetal well-being include sonographic assessment of fetal growth and amniotic fluid volume, umbilical artery Doppler, nonstress test (NST), and biophysical profile (BPP). NST and Doppler are considered more predictive of fetal well-being than the other two, because of the inaccuracy of amniotic fluid volume estimation in multiple gestations.

One study showed that growth restriction confers worse perinatal outcome in twins when compared with singletons. Serial US scans to assess fetal weight and growth should be done every 3-4 weeks from 18 to 20 weeks until delivery. Others advocate follow-up beginning at 28 weeks since fetal growth in twin pregnancies mirrors singleton growth until 28 to 32 weeks. There are others that advocate the use of twin growth charts in the third trimester. Fetal growth in twin and multifetal pregnancies after 30 weeks of gestation lags behind the growth of singleton pregnancies. Therefore, 2- to 4-week intervals for serial US scans to monitor interval growth in the third trimester has been recommended.

If growth discordance (>20% to 30%, calculated as a percentage of the larger twin's weight) or growth restriction (<10%) is discovered, US is recommended every 2 weeks. Optimal detection of fetal growth deceleration leading to growth discordance is between 20 and 28 weeks. Doppler testing has been validated in multifetal gestations; the values and patterns of change in vascular resistance are the same as for singletons.

Weekly surveillance for all multifetal pregnancies has not been validated in prospective studies. Surveillance with nonstress testing or biophysical profile for pregnancies complicated by abnormal fluid volumes, pregnancy-induced hypertension, fetal anomalies, growth abnormalities, monoamnionicity, or other standard obstetric indications is as reliable in multiple gestations as in singleton gestations.

The necessary parameters to measure or calculate in assessing the likelihood of intrauterine growth restriction include weight and abdominal circumference. Measurements of biparietal diameter, head circumference, and femur length are all indicated, but ratios of head or femur to abdominal circumference are probably not needed. The use of the same measurement tables developed for singleton pregnancies is indicated for twins, rather than tables specifically generated for twins. Twin pregnancies are at greater risk of intrauterine growth restriction, which may affect one or both fetuses, and there is concern that growth tables for twins, which do show smaller measurements than singletons in the third trimester, may be incorporating tendencies toward growth restriction within their normal values. It is important to remember that twins can be concordantly growth-restricted, and if both are becoming small for dates on follow-up sonograms, protocols for monitoring fetal well-being will still be indicated, just as they would be in significantly discordant twins.

Discordant fetal growth is seen in a significant proportion of twin pregnancies. The abdominal circumference ratio has been shown to be a good predictor of birth weight discordance, and it can be added to the follow-up protocol. Discordance is considered mild if weight estimates for the twins are 15% different, moderate if 20% different, and severe if 25% different or greater. For mild discordance, scans for growth at 3-week intervals with use of umbilical artery Doppler (UAD) analysis are probably indicated. For moderate discordance, scans for growth at 2-3-week intervals should be considered, and UAD, BPP, and/or NST are indicated. When discordance is severe, growth scans at 2-week intervals are preferred, with BPP and/or NST necessary and UAD also indicated. If both twins have fallen below the 10th percentile for gestational age relative to menstrual dates and/or dating by the initial sonogram, that should also be taken as an indication for increased surveillance of growth and fetal health.

Oligohydramnios in one or both sacs may indicate uteroplacental insufficiency necessitating further testing to screen for fetal well-being. It may also be an early sign of TTTS if the other twin is presenting with polyhydramnios. If deemed abnormal, fetal well-being scans (which may include BPP, NTS, and UAD) are recommended every 2 weeks. Therefore amniotic fluid determination is another standard parameter evaluated during the routine sonographic examination.

UAD is, in general, not a rapidly fluctuating or deteriorating parameter, but rather a long-term predictor of the status of the uteroplacental circulation. As such, it has prognostic significance for the likelihood of growth restriction and perinatal morbidity and mortality, and it may change weekly if abnormal. Doppler assessment of umbilical venous blood flow and systolic/diastolic ratios is considered useful in predicting and confirming concordant and discordant growth in twins. Absent end diastolic flow is associated with low birth weight, growth restriction, and perinatal mortality in triplet and quadruplet pregnancies.

On each indicated follow-up sonogram, it remains equally important to measure twins for development of discordance, and to evaluate the cervix and each twin's amniotic fluid. Transvaginal sonography has been shown to be able to accurately assess cervical length and predict spontaneous delivery before 34 weeks as early as the 27th or even the 23rd week. A large multicenter study of cervical length in twin pregnancies determined that a cervical length of less than 25 mm at 24 weeks gestation was the best predictor of delivery before 32 weeks of gestation and was significantly more common in the 24th and 28th weeks of gestation in twin gestations compared with singletons.

Approximately 10%-20% of monochorionic twin pregnancies may be associated with TTTS, a type of twin discordance. In addition to the evaluation of amniotic fluid volume, bladder volume, and hydrops in each twin, Doppler findings may be used to assess these pregnancies complicated by intertwin vascular connections within the placenta, between cord insertion sites, for the presence of an arterio-arterial anastomosis. Other Doppler findings include absent or reversed end-diastolic flow within the umbilical artery, pulsatile umbilical vein flow, and absent or reversed end-diastolic flow within the ductus venosus.

The most effective fetal surveillance system for multiple gestations is currently not known. The gestational age at which testing should be initiated is still not established. It is also still unclear whether testing should be performed once or more than once per week or whether there is a need to test normally growing dichorionic twins. There are no current studies that prove that routine antepartum surveillance provides objective benefit in the absence of other high-risk conditions. At present, antepartum fetal testing in multiple gestations is recommended in all situations in which surveillance would ordinarily be performed in a singleton pregnancy (including in utero growth retardation). In the most recent practice bulletin of the American College of Obstetricians and Gynecologists, the recommendation based on consensus and expert opinion was that the management of discordant growth restriction or death of one fetus in a high-order multiple gestation should be individualized, taking into consideration the welfare of the other fetuses.

Summary

The evaluation of multiple gestations is a challenging and important task. The intensity of the obstetrical management of such pregnancies must be titrated to the degree of risk present in each individual case. The number of fetuses present, their chorionic and amniotic status, and risk factors such as growth restriction of one or more fetuses, amniotic fluid alterations, or presence of fetal anomalies must all be taken into account. These parameters will all affect the frequency of growth assessment, the intensity of surveillance for fetal well-being, and the institution of pharmacological and other medical therapeutic interventions. Ultrasonographic imaging, together with its associated techniques for monitoring fetal compensation or distress, serves as the mainstay for evaluating the complexities of each multifetal pregnancy, helping the obstetrician chart a course toward a successful outcome.

None provided

The recommendations are based on analysis of the current literature and expert panel consensus.

• Allison SO, Andreotti RF, Lee SI, Angtuaco TL, Horrow MM, Javitt MC, Lev-Toaff AS, Podrasky AE, Scoutt LM, Zelop C, Expert Panel on Women's Imaging. ACR Appropriateness Criteria® multiple gestations. [online publication]. Reston (VA): American College of Radiology (ACR); 2008. 6 p. [46 references]

Not applicable: The guideline was not adapted from another source.

1996 (revised 2008)

American College of Radiology - Medical Specialty Society

The American College of Radiology (ACR) provided the funding and the resources for these ACR Appropriateness Criteria®.

Committee on Appropriateness Criteria, Expert Panel on Women's Imaging

Panel Members: Sandra O. DeJesus Allison, MD; Rochelle F. Andreotti, MD; Susanna I. Lee MD, PhD; Teresita L. Angtuaco, MD; Mindy M. Horrow, MD; Marcia C. Javitt, MD; Anna S. Lev-Toaff, MD; Ann E. Podrasky, MD; Leslie M. Scoutt, MD; Carolyn Zelop, MD

Not stated

This is the current release of the guideline.

This guideline updates a previous version: Fleischer AC, Andreotti RF, Bohm-Velez M, Horrow MM, Hricak H, Javitt MC, Thurmond A, Zelop C, Expert Panel on Women's Imaging. Multiple gestations. [online publication]. Reston (VA): American College of Radiology (ACR); 2005. 10 p.

The appropriateness criteria are reviewed annually and updated by the panels as needed, depending on introduction of new and highly significant scientific evidence.

None available

This NGC summary was completed by ECRI on November 19, 2004. The information was verified by the guideline developer on December 21, 2004. This summary was updated by ECRI on March 23, 2006. This NGC summary was updated by ECRI Institute on August 11, 2009.