Fetal biometry and outcome in monochorionic vs. dichorionic twin pregnancies; a retrospective cross-sectional matched-control study

Ultrasound for diagnosis of birth weight discordance in twin pregnancies

BACKGROUND

There is a need to standardize monitoring in obstetric research of twin pregnancies. Identification of birth weight discordance (BWD), defined as a difference in the birth weights of twins, is a well-documented phenomenon in twin pregnancies. Ultrasound for the diagnosis of BWD informs complex decision making including whether to intervene medically (via laser photo coagulation) or deliver the twins to avoid fetal morbidities or even death. The question is, how accurate is this measurement?

OBJECTIVES

To determine the diagnostic accuracy (sensitivity and specificity) of ultrasound estimated fetal weight discordance (EFWD) of 20% and 25% using different estimated biometric ultrasound measurements compared with the actual BWD as the reference standard in twin pregnancies.

SEARCH METHODS

The search for this review was performed on 15 March 2019. We searched CENTRAL, MEDLINE (Ovid), Embase (Ovid), seven other databases, conference proceedings, reference lists and contacted experts. There were no language or date restrictions applied to the electronic searches, and no methodological filters to maximize sensitivity.

SELECTION CRITERIA

We selected cohort-type studies with delayed verification that evaluated the accuracy of biometric measurements at ultrasound scanning of twin pregnancies that had been proposed for the diagnosis of estimated BWD, compared to BWD measurements after birth as a reference standard. In addition, we only selected studies that considered twin pregnancies and applied a reference standard for EFWD for the target condition of BWD.

DATA COLLECTION AND ANALYSIS

We screened all titles generated by electronic database searches. Two review authors independently assessed the abstracts of all potentially relevant studies. We assessed the identified full papers for eligibility, and extracted data to create 2 × 2 tables. Two review authors independently performed quality assessment using the QUADAS-2 tool. We excluded studies that did not report data in sufficient detail to construct 2 × 2 tables, and where this information was not available from the primary investigators. We assessed the certainty of the evidence using the GRADE approach.

MAIN RESULTS

We included 39 eligible studies with a median study sample size of 140. In terms of risk of bias, there were many unclear statements regarding patient selection, index test and use of proper reference standard. Twenty-one studies (53%) were of methodological concern due to flow and timing. In terms of applicability, most studies were of low concern. Ultrasound for diagnosis of BWD in twin pregnancies at 20% cut-off Twenty-two studies provided data for a BWD of 20% and the summary estimate of sensitivity was 0.51 (95% CI 0.42 to 0.60), and the summary estimate of specificity was 0.91 (95% CI 0.89 to 0.93) (8005 twin pregnancies; very low-certainty evidence). Ultrasound for diagnosis of BWD in twin pregnancies at 25% cut-off Eighteen studies provided data using a BWD discordance of 25%. The summary estimate of sensitivity was 0.46 (95% CI 0.26 to 0.66), and the summary estimate of specificity was 0.93 (95% CI 0.89 to 0.96) (6471 twin pregnancies; very low-certainty evidence). Subgroup analyses were possible for both BWD of 20% and 25%. The diagnostic accuracy did not differ substantially between estimation by abdominal circumference and femur length but femur length had a trend towards higher sensitivity and specificity. Subgroup analyses were not possible by sex of twins, chorionicity or gestational age due to insufficient data.

AUTHORS' CONCLUSIONS

Very low-certainty evidence suggests that EFWD identified by ultrasound has low sensitivity but good specificity in detecting BWD in twin pregnancies. There is uncertain diagnostic value of EFWD; this review suggests there is insufficient evidence to support this index as the sole measure for clinical decision making to evaluate the prognosis of twins with growth discordance. The diagnostic accuracy of other measures including amniotic fluid index and umbilical artery Doppler resistive indices in combination with ultrasound for clinical intervention requires evaluation. Future well-designed studies could also evaluate the impact of chorionicity, sex and gestational age in the diagnostic accuracy of ultrasound for EFWD.

Birth management and fetal outcome in multiple gestation: analysis of 1.444 births

PURPOSE

Since the late 1990s, controversies came up concerning the mode of delivery for multiples births. The twin birth study indicated no difference in maternal and fetal outcome between planned vaginal delivery and planned caesarean section, but did not analyze the influence of maternal and fetal characteristics or the obstetric management in vaginal birth. The purpose of this study is to analyze these parameters regarding fetal outcome.

METHODS

A large-scale retrospective analysis of twin births (n = 1.444) was performed at a university medical center. The analysis included pregnancy, delivery, and maternal and fetal parameters, including pH and base excess (BE) differences between the first- and second-born twin (delta pH, delta BE).

RESULTS

Delta pH correlated significantly with the birth interval for various positions of twins in the womb (p < 0.05). The longer the birth interval, the greater the delta pH with a lower pH of the second twin. Delta BE values were significantly correlated with the birth interval for a combination of twins in cephalic and breech presentation. Furthermore, it could be shown that higher differences in birth weight between the first/second twin are associated with higher delta pH, higher delta BE values (all p < 0.05). We found significantly decreasing delta pH values in vaginal deliveries over secondary and, finally, primary caesarean sections (p < 0.001).

CONCLUSION

We conclude a vaginal delivery of twin appears safe if experienced staff monitor birth weight discrepancies, birth interval, and blood values consequently. A good outcome also for the second twin delivered spontaneously is nevertheless feasible if experienced staff is available.

A systematic review of amniotic fluid assessments in twin pregnancies

The objectives of this systematic review were to examine the reproducibility of sonographic estimates of amniotic fluid volume (AFV) in twin pregnancies, compare the association of sonographic estimates of AFV with dye-determined AFV, and correlate AFV with antepartum, intrapartum, and perinatal outcomes in twin pregnancies. Studies were included if they were adequately powered and investigated antepartum, intrapartum, and/or perinatal adverse outcome parameters in twin gestations. Studies with comparable populations and exclusion criteria were merged into forest plots. Data comparing the accuracy of AFV assessment, correlation of AFV with gestational age, and adverse outcomes were tabulated. Five of the 6 studies investigating AFV by the amniotic fluid index as a function of gestational age reported data fitting a quadratic equation, with fluid volumes peaking at mid gestation and then declining. This trend was less pronounced when AFV was assessed by the single deepest pocket (2 of 4 studies reporting a quadratic fit). Polyhydramnios was associated with prematurity in 2 of 4 studies (1 amniotic fluid index and 1 single deepest pocket), and oligohydramnios was associated with prematurity in 1 single deepest pocket study. Stillbirth was the only intrapartum outcome reported in more than 1 study. Perinatal outcomes associated with polyhydramnios included neonatal death (P < .05 in 1 of 2 studies), low Apgar scores (1 of 2 studies), neonatal intensive care unit admission (1 of 2 studies), and low birth weight (2 of 3 studies).

Ultrasound in twin pregnancies

OBJECTIVE

To review the literature with respect to the use of diagnostic ultrasound in the management of twin pregnancies. To make recommendations for the best use of ultrasound in twin pregnancies.

OUTCOMES

Reduction in perinatal mortality and morbidity and short- and long-term neonatal morbidity in twin pregnancies. Optimization of ultrasound use in twin pregnancies.

EVIDENCE

Published literature was retrieved through searches of PubMed and the Cochrane Library in 2008 and 2009 using appropriate controlled vocabulary (e.g., twin, ultrasound, cervix, prematurity) and key words (e.g., acardiac, twin, reversed arterial perfusion, twin-to-twin transfusion syndrome, amniotic fluid). Results were restricted to systematic reviews, randomized control trials/controlled clinical trials, and observational studies. There were no date restrictions. Studies were restricted to those with available English or French abstracts or text. Searches were updated on a regular basis and incorporated into the guideline to September 2009. Grey (unpublished) literature was identified through searching the websites of health technology assessment and health technology assessment-related agencies, clinical practice guideline collections, clinical trial registries, and national and international medical specialty societies.

VALUES

The evidence collected was reviewed by the Diagnostic Imaging Committee of the Society of Obstetricians and Gynaecologists of Canada, with input from members of the Maternal Fetal Medicine Committee and the Genetics Committee of the SOGC. The recommendations were made according to the guidelines developed by The Canadian Task Force on Preventive Health Care (Table 1).

BENEFITS, HARMS, AND COSTS

The benefit expected from this guideline is facilitation and optimization of the use of ultrasound in twin pregnancy. SUMMARY STATEMENTS: 1. There are insufficient data to make recommendations on repeat anatomical assessments in twin pregnancies. Therefore, a complete anatomical survey at each scan may not be needed following a complete and normal assessment. (III) 2. There are insufficient data to recommend a routine preterm labour surveillance protocol in terms of frequency, timing, and optimal cervical length thresholds. (II-2) 3. Singleton growth curves currently provide the best predictors of adverse outcome in twins and may be used for evaluating growth abnormalities. (III) 4. It is suggested that growth discordance be defined using either a difference (20 mm) in absolute measurement in abdominal circumference or a difference of 20% in ultrasound-derived estimated fetal weight. (II-2) 5. Although there is insufficient evidence to recommend a specific schedule for ultrasound assessment of twin gestation, most experts recommend serial ultrasound assessment every 2 to 3 weeks, starting at 16 weeks of gestation for monochorionic pregnancies and every 3 to 4 weeks, starting from the anatomy scan (18 to 22 weeks) for dichorionic pregnancies. (II-1) 6. Umbilical artery Doppler may be useful in the surveillance of twin gestations when there are complications involving the placental circulation or fetal hemodynamic physiology. (II-2) 7. Although many methods of evaluating the level of amniotic fluid in twins (deepest vertical pocket, single pocket, amniotic fluid index) have been described, there is not enough evidence to suggest that one method is more predictive than the others of adverse pregnancy outcome. (II-3) 8. Referral to an appropriate high-risk pregnancy centre is indicated when complications unique to twins are suspected on ultrasound. (II-2) These complications include: 1. Twin-to-twin transfusion syndrome 2. Monoamniotic twins gestation 3. Conjoined twins 4. Twin reversed arterial perfusion sequence 5. Single fetal death in the second or third trimester 6. Growth discordance in monochorionic twins. Recommendations 1. All patients who are suspected to have a twin pregnancy on first trimester physical examination or who are at risk (e.g., pregnancies resulting from assisted reproductive technologies) should have first trimester ultrasound performed. (II-2A) 2. Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified. (II-2A) 3. Although the accuracy in confirmation of gestational age at the first and second trimester is comparable, dating should be done with first trimester ultrasound. (II-2A) 4. Beyond the first trimester, it is suggested that a combination of parameters rather than a single parameter should be used to confirm gestational age. (II-2C) 5. When twin pregnancy is the result of in vitro fertilization, accurate determination of gestational age should be made from the date of embryo transfer. (II-1A) 6. There is insufficient evidence to make a recommendation of which fetus (when discordant for size) to use to date a twin pregnancy. However, to avoid missing a situation of early intrauterine growth restriction in one twin, most experts agree that the clinician may consider dating pregnancy using the larger fetus. (III-C) 7. In twin pregnancies, aneuploidy screening using nuchal transluscency measurements should be offered. (II-2B) 8. Detailed ultrasound examination to screen for fetal anomalies should be offered, preferably between 18 and 22 weeks' gestation, in all twin pregnancies. (II-2B) 9. When ultrasound is used to screen for preterm birth in a twin gestation, endovaginal ultrasound measurement of the cervical length should be performed. (II-2A) 10. Increased fetal surveillance should be considered when there is either growth restriction diagnosed in one twin or significant growth discordance. (II-2A) 11. Umbilical artery Doppler should not be routinely offered in uncomplicated twin pregnancies. (I-E) 12. For defining oligohydramnios and polyhydramnios, the ultrasonographer should use the deepest vertical pocket in either sac: oligohydramnios when < 2 cm and polyhydramnios when > 8 cm. (II-2B).

Optimum timing for planned delivery of uncomplicated monochorionic and dichorionic twin pregnancies

OBJECTIVE

To determine the optimum timing for planned delivery of uncomplicated monochorionic and dichorionic twin pregnancies.

METHODS

Unselected twin pregnancies were recruited for this prospective cohort study (N=1,028), which was conducted in eight tertiary referral perinatal centers in Ireland. Perinatal mortality and a composite measure of perinatal morbidity (respiratory distress, necrotizing enterocolitis, hypoxic ischemic encephalopathy, periventricular leukomalacia, or sepsis) were compared between uncomplicated twins that underwent planned preterm delivery compared with monochorionic twins that continued in utero beyond 34 weeks of gestation, and dichorionic twins who continued beyond 36 weeks.

RESULTS

Perinatal outcome data were recorded for 100% of the 1,001 twin pairs that completed the study (n=200 monochorionic and n=801 dichorionic). Overall perinatal mortality was 30 per 1,000 in monochorionic twins and 3.8 per 1,000 among dichorionic twins. The prospective risk of in utero death was 1.5% after 34 weeks of gestation for uncomplicated monochorionic pregnancies, with no deaths among dichorionic twins after 33 weeks. The risk of a composite measure of perinatal morbidity for uncomplicated monochorionic twins fell from 41% (13/32 neonates, 3/6 among elective deliveries) at 34 weeks to 5% (4/84) at 37 weeks (P<.001). Among dichorionic twins, the risk of morbidity fell from 4% (2/52) among elective deliveries at 36 weeks to 1% (5/344) in pregnancies continuing to 38 weeks (P=.231).

CONCLUSION

Applying a strategy of close fetal surveillance, perinatal morbidity can be minimized by allowing uncomplicated monochorionic pregnancies continue to 37 weeks of gestation and dichorionic twins to 38 weeks. Among monochorionic twins, this approach must be balanced against a 1.5% risk of late in utero death.

The Natural Course of Monochorionic and Dichorionic Twin Pregnancies: A Historical Cohort

Current early diagnosis, surveillance and intervention options make it hard to determine the natural course of twin pregnancies, especially regarding spontaneous preterm delivery and perinatal mortality. We studied the natural course in monochorionic (MC) and dichorionic (DC) twin pregnancies in a historical cohort. Twin pregnancies were studied in a unique database of 651 twin pairs born in the period 1907 to 1938. We examined the effect of chorionicity on gestational age, birthweight, peri-natal mortality, intertwin birthweight differences, the incidence of preeclampsia and maternal mortality. Perinatal mortality was 27.7% for MC and 15.8% for DC twins (p < .001). Gestational age and birthweight were stronger predictors of perinatal mortality than chorionicity. Perinatal outcome was poorer for the second twin, especially in DC twins. Delivery before 37 weeks of gestation occurred more often in MC twin pregnancies (48.8% compared to 33.3% in DC twin pregnancies). DC twins were on average 288 g (95% confidence interval 201–376) heavier than MC twins. Severe birthweight discordancy occurred equally in MC and DC twins (18.1%). However, if present, mortality was only increased in MC twins. The birthweight of girls was not affected by the presence of a male co-twin. In this historical cohort MC twin pregnancies had a higher perinatal mortality, caused by a high incidence of low birthweight mainly due to preterm delivery. Mortality did not differ in deliveries after 31 weeks of gestation, which is in contrast to recent data. Apparently, modern obstetrics is more effective in reducing mortality in DC twins.

Prediction of safe and successful vaginal twin birth

OBJECTIVE

The objective of the study was to establish predictors of vaginal twin birth and evaluate perinatal morbidity according to mode of delivery.

STUDY DESIGN

One thousand twenty-eight twin pregnancies were prospectively recruited. For this prespecified secondary analysis, obstetric characteristics and a composite of adverse perinatal outcome were compared according to the success or failure of a trial of labor and further compared with those undergoing elective cesarean delivery. Perinatal outcomes were adjusted for chorionicity and gestational age using a linear model for continuous data and logistic regression for binary data.

RESULTS

Nine hundred seventy-one twin pregnancies met the criteria for inclusion. A trial of labor was considered for 441 (45%) and was successful in 338 of 441 (77%). The cesarean delivery rate for the second twin was 4% (14 of 351). Multiparity and spontaneous conception predicted vaginal birth. No statistically significant differences in perinatal morbidity were observed.

CONCLUSION

A high prospect of successful and safe vaginal delivery can be achieved with trial of twin labor.

Stillbirth and neonatal mortality in monochorionic and dichorionic twins: a population-based study

BACKGROUND

Chorionicity is one of the main predictors of higher perinatal mortality in twins. The aim of this large population-based study was to analyse stillbirth and neonatal mortality by cause of death and chorionicity and to quantify the risk of stillbirth by gestational age in dichorionic (DC) and monochorionic (MC) twins.

METHODS

We used data on twin maternities delivered in the North of England from 1998 to 2007 and notified to the Northern Survey of Twin and Multiple Pregnancy. Prospective risk of stillbirth by gestational age at death was calculated using number of stillborn fetuses at or beyond a given gestational period per 1000 fetuses in ongoing pregnancies.

RESULTS

There were 4565 twin maternities (9130 twins) with an overall twinning rate of 14.9 per 1000 maternities. The overall stillbirth and neonatal mortality rates in twins during 1998-2007 were 18.0/1000 births and 23.0/1000 live births, respectively. Stillbirth and neonatal mortality rates were significantly higher in MC than DC twins: 44.4 versus 12.2 per 1000 births [relative risk (RR): 3.6; 95% CI: 2.6-5.1], and 32.4 versus 21.4 per 1000 live births (RR: 1.5; 95% CI: 1.04-2.2), respectively. There was no significant improvement over time in either stillbirth or neonatal mortality rates in either group. The prospective risk of antepartum stillbirth was higher for MC than DC twins at all preterm gestations and the highest risk was before 28 weeks' gestation.

CONCLUSIONS

MC twins have higher rates of stillbirth and neonatal mortality than DC twins, and rates did not improve over 1998-2007. The prospective risk of antepartum stillbirth is much higher for MC twins at all gestational ages.

First-trimester ultrasound examination and the outcome of monochorionic twin pregnancies

OBJECTIVE

To refine the incidence of abnormal first-trimester ultrasound measurements and their correlation with the outcome of monochorionic diamniotic pregnancies.

METHODS

First-trimester crown-rump length (CRL) and nuchal translucency thickness (NT) measurements were studied in three subgroups of a total of 200 monochorionic twin gestations referred to our center between June 2002 and February 2006. Intertwin CRL discordance was defined as > 10% and the 95th percentile of NT thickness for gestational age was used. The first group of 103 consecutive unselected monochorionic diamniotic twin pregnancies was prospectively followed up from 11-14 weeks onwards, throughout the pregnancy. The second group of 136 nonconsecutive monochorionic diamniotic twin pregnancies including 64 that developed TTTS was studied retrospectively. The third group of 100 consecutive cases of TTTS studied retrospectively for the correlation between first trimester measurements and staging and timing of occurrence of TTTS.

RESULTS

In group 1, the incidence of TTTS was 5 in 103 (5%, 95CI [0.7-9]). Large intertwin CRL discordance and increased NT were correlated with perinatal death. In group 2, no significant association was found between first-trimester parameters and the development of TTTS but discordance in early second trimester biometry and Doppler were. In group three, a positive correlation was found between the intertwin discordance in CRL and early occurrence of TTTS before 20 weeks of gestation (p = 0.02).

CONCLUSION

Monochorionic twin gestations who ultimately develop TTTS may exhibit intertwin difference in growth as early as 11-14 weeks of gestation. The earlier the discordance the earlier the development of the disease.

Increased perinatal mortality and morbidity in monochorionic versus dichorionic twin pregnancies: clinical implications of a large Dutch cohort study

OBJECTIVE

To evaluate mortality and morbidity in a large cohort of twin pregnancies according to chorionicity. We aimed to estimate the optimal time of delivery.

DESIGN

Historical cohort design. Setting Two teaching hospitals.

POPULATION

Twin pregnancies delivered in the University Medical Centre, Utrecht, and the St Elisabeth Hospital, Tilburg (1995-2004), The Netherlands (n = 1407).

METHODS

Pregnancy outcomes were documented according to chorionicity. Mortality >/=32 weeks was reviewed carefully with special attention to antenatal fetal monitoring, autopsy and placental histopathology to find an explanation for adverse outcome.

MAIN OUTCOME MEASURES

Perinatal mortality and morbidity in monochorionic (MC) and dichorionic (DC) twins.

RESULTS

Perinatal mortality was 11.6% in MC twin pregnancies and 5.0% in DC twin pregnancies. After 32 weeks, the risk of intrauterine death (IUD) was significantly higher in MC twins than in DC twins (hazard ratio 8.8, 95% CI 2.7-28.9). In most of these cases of IUD, no antenatal signs of impaired fetal condition had been present. Median gestational age was 1 week longer in DC twins than in MC twins, and the mean birthweight was 221 g higher. Severe birthweight discordancy (>20%) occurred more often in MC twins than in DC twins (OR 1.23, 95% CI 0.97-1.55). The incidence of necrotising enterocolitis (NEC) was higher in MC twins, after adjustment for age and weight at birth (OR 4.05, 95% CI 1.97-8.35). There was a trend towards higher neuromorbidity in MC twins.

CONCLUSIONS

This is the largest cohort study of twin pregnancies evaluating outcome according to chorionicity thus far. MC twins are at increased risk for fetal death (even at term), NEC and neuromorbidity. Current antenatal care is insufficient to predict and prevent this excess perinatal mortality and morbidity. Planned delivery at or even before 37 weeks of gestation seems to be justified for MC twins.

[Fetal growth in multiple pregnancy]

A close surveillance of fetal growth in multiple pregnancies is mandatory for the prevention and/or reduction of neonatal morbidity and mortality. Multiples have the same genetically determined growth potential as singletons. However, this growth potential is restricted by the functional capacity of the placenta and uterus. Multiples show a specific growth pattern compared to singletons. Nevertheless, for clinical surveillance, the 10th percentile of singleton growth charts may be used, because significant differences, which may define normally grown multiples as small for gestational age or growth restricted, can only be detected at higher gestational ages, when most of the multiples may have already been delivered. At higher gestational ages, obstetricians should take into account the specific growth pattern for multiples. In multiples with growth discordance, it is necessary to consider or exclude the various causes, and if necessary the clinical follow-up has to be intensified.

The Northwestern twin chorionicity study: testing the 'placental crowding' hypothesis

OBJECTIVE

To evaluate the relation between placental proximity and frequency of birth weight discordance and small-for-gestational age (SGA) infants.

STUDY DESIGN

Retrospective three-tier chorionicity analysis of 1155 twin placentas comparing birth weight characteristics of the twins in different placental types.

RESULTS

Dichorionic-separate, but not dichorionic-fused twins, are heavier than monochorionic-diamniotic and monoamniotic twins (2376+/-721 vs. 2274+/-770, P < 0.006, and 2376+/-721 vs. 2166+/-782, P < 0.04). SGA twins are less frequent among dichorionic twins (OR 0.4; 95% CI 0.3, 0.6). Fewer sets with two SGA infants are present among dichorionic-separate compared to monochorionic-diamniotic pairs (OR 0.3; 95% CI 0.1, 0.8). The same trends are found when comparing all dichorionic to all monochorionic twins. Twins of all placental types have similar gestational ages and discordance values.

CONCLUSIONS

Dichorionic-separate placentas are least likely to experience 'placental crowding' and thus are associated with heavier twins and fewer sets with one or two SGA infants.

Persistance of adverse obstetric and neonatal outcomes in monochorionic twins after exclusion of disorders unique to monochorionic placentation

OBJECTIVE

This study was undertaken to assess obstetric and neonatal outcomes in dichorionic twins and monochorionic-diamniotic twins after exclusion of twin-to-twin transfusion syndrome and twin reversed arterial perfusion sequence.

STUDY DESIGN

Data from a tertiary center were collected in twin gestations between 1994 and 2002. Chorionicity was defined by standard echographic criteria and placental examination at delivery. Neonatal outcomes were compared between monochorionic and dichorionic gestations.

RESULTS

This study included 503 women: 378 (75%) dichorionic and 125 (25%) monochorionic twin gestations. Monochorionic twin gestations had a higher risk of preterm deliveries between 30 and 34 weeks' gestation than pregnancies with dichorionic twins (P < .01). Monochorionic twins had a higher number of birth weight less than 10th percentile (P < .001) discordancy 25% or greater (P < .02), admission to neonatal intensive care unit (P < .03), and intraventricular hemorrhage grade 3 and 4 (P < .007) than dichorionic twins even after adjusting for gestational age.

CONCLUSION

Monochorionic diamniotic twins have a higher risk of perinatal complications than dichorionic twin gestations, even after exclusion of disorders unique to monochorionic placentation.

Is zygosity or chorionicity the main determinant of fetal outcome in twin pregnancies?

OBJECTIVE

The purpose of this study was to examine whether fetal outcome in twin pregnancies is dependent on zygosity or chorionicity.

STUDY DESIGN

This was a prospective observational study comprised of women with twin pregnancies who attended the fetal medicine unit at St Michael's Hospital, Bristol, Ireland, during the years 1998 to 2000 and who were delivered in hospitals in south west England. After delivery, zygosity was determined with umbilical cord blood with the use of microsatellite markers that were amplified by polymerase chain reaction. Placentae were examined histologically for chorionic type. The perinatal outcomes of 3 groups of monozygotic monochorionic, monozygotic dichorionic, and dizygotic pregnancies were compared with the use of the Mann-Whitney U test and the Fisher's exact test.

RESULTS

All 92 dizygotic and 15 monozygotic dichorionic pregnancies resulted in live births. In 7 of the 39 cases in the monozygotic monochorionic group, either both twins were not live born or delivery occurred <24 weeks of gestation. The gestational age at delivery and birth weight were significantly lower, and there were a greater number of cases with birth weight discordancy of >25% in the monochorionic pregnancies compared with the other 2 groups (P < .05). There were no significant differences in any of the study parameters between the monozygotic dichorionic and dizygotic groups.

CONCLUSION

Fetal outcome in twin pregnancies is related to chorionicity rather than zygosity.

Growth Aberration in Multiple Pregnancy

Growth of twins and higher-order multiples is an exceptional metabolic challenge for the expecting mother. She is doing much more than a mother of a singleton in terms of nurturing, however. Metabolic requirements need adequate dietary intervention in the form of increased weight gain during early pregnancy. It is normal for multiples to be smaller than singletons. Being smaller than singletons does not necessarily mean that multiples are pathologically growth restricted. It is important to remember that twins and triplets have different growth patterns, and their growth should not be considered by using singleton standards. When a small-for-gestational-age fetus is suspected in a multiple pregnancy, it is advisable to follow or to treat the pregnancy as if it was an SGA singleton.

Does chorionicity or zygosity predict adverse perinatal outcomes in twins?

OBJECTIVE

The purpose of this study was to evaluate chorionicity and zygosity as risk factors for adverse perinatal outcomes in twins.

STUDY DESIGN

A population-based, retrospective cohort study was conducted of all twin deliveries in Nova Scotia, Canada, from 1988 to 1997. Chorionicity was established by histologic examination. Zygosity was determined by chorionicity, sex, and infant blood group. Three groups were established: monochorionic/monozygotic twins, dichorionic/dizygotic twins, and dichorionic/majority monozygotic twins.

RESULTS

Outcomes from 1008 twin pregnancies were analyzed. Monochorionic/monozygotic twins had lower mean birth weights compared with dichorionic/dizygotic twins. Rates of perinatal mortality of at least 1 twin were significantly higher among monochorionic/monozygotic twins relative to dichorionic/dizygotic twins (relative risk, 2.5; 95% CI, 1.1-2.5). Dichorionic/majority monozygotic twins had similar perinatal outcomes compared with dichorionic/dizygotic twins.

CONCLUSION

Monochorionicity increases the risk of adverse perinatal outcome, whereas the effect of zygosity is less clear. Because chorionicity can be determined by prenatal ultrasound scanning, this information should be considered in the prenatal care of twin pregnancies.

Twin pregnancy: the role of ultrasound in management

Determination of chorionicity is one of the most important issues in the management of twin pregnancy. Modern ultrasound equipment has made it possible to accurately assess placentation already in the first trimester with the lambda sign. With regard to prenatal diagnosis, it is important to know the chorionicity in order to calculate the risk of chromosomally abnormal fetuses. Accurate chorionicity offers the obstetricians the opportunity to observe the monochorionic twins more intensively than is required for twins with dichorionic placentation. This review gives an update of the state of the art for clinicians caring for twin pregnancies.

Twin pregnancy

Recent advances regarding twin pregnancies have focused on several problems, including the detection and definition of risk in relation to chorionicity, the management of a pregnancy with a single anomalous fetus, the prediction of prognosis and the management of twin-twin transfusion syndrome, and the detection of preterm labor. These questions will be considered in this review.