Objective
The purpose of this study was to evaluate the impact of noncentral placental cord insertion on birthweight discordance in twins.
Study Design
We performed a multicenter, prospective trial of twin pregnancies. Placental cord insertion was documented as central, marginal, or velamentous according to a defined protocol. Association of the placental cord insertion site with chorionicity, birthweight discordance, and growth restriction were assessed.
Results
Eight hundred sixteen twin pairs were evaluated; 165 pairs were monochorionic, and 651 pairs were dichorionic. Monochorionic twins had higher rates of marginal ( P = .0068) and velamentous ( P < .0001) placental cord insertion. Noncentral placental cord insertion was more frequent in smaller twins of discordant pairs than control pairs (29.8% vs 19.1%; P = .004). Velamentous placental cord insertion in monochorionic twins was associated significantly with birthweight discordance (odds ratio, 3.5; 95% confidence interval, 1.3–9.4) and growth restriction (odds ratio, 4; 95% confidence interval, 1.1–14.3).
Conclusion
Noncentral placental cord insertion contributes to birthweight discordance in monochorionic twin pregnancies. Sonographic delineation of placental cord insertion may be of value in antenatal assessment of twin pregnancies.
Twin pregnancies are associated with increased perinatal mortality and morbidity rates. Although much of this increase in morbidity is a consequence of the increased rates of preterm delivery in twin pregnancies, there is also an independent association with growth abnormalities. Studies that have evaluated perinatal outcome in twins have reported excess morbidity and death at increasing levels of birthweight discordance.
The higher rates of adverse neonatal outcomes that are associated with discordant growth occur irrespective of gestational age at delivery and independent of small-for-gestational-age (SGA) status. Greater degrees of growth discordance have been shown to increase the risk of intrauterine death for both the smaller and larger twin.
Placental umbilical cord insertion site has been evaluated as a contributory factor to perinatal morbidity in singleton and twin pregnancies. In singleton pregnancies, a velamentous cord insertion is associated with obstetric complications that include prematurity, congenital anomalies, and fetal growth restriction. A velamentous insertion is found in approximately 1% of singleton pregnancies, with an additional 7% of singleton pregnancies having a marginal cord insertion.
Noncentral cord insertion is more common in twin than in singleton pregnancies. In particular, monochorionic twin pregnancies have significantly higher rates of velamentous cord insertion. This has been proposed as a contributory factor in the development of selective intrauterine growth restriction in monochorionic twin pregnancies; some studies show conflicting results with respect to the role of velamentous cord insertion in the cause of twin-to-twin transfusion syndrome (TTTS).
The aim of this study was to evaluate the relative frequency of noncentral cord insertion in monochorionic and dichorionic twin pregnancies and to examine the association between noncentral cord insertion and birthweight discordance in twins.
Materials and Methods
This study was performed as a prespecified secondary analysis of the Evaluation of Sonographic Predictors of Restricted Growth in Twins (ESPRiT) study. ESPRiT was a multicenter, prospective, observational study of twin pregnancies that was carried out at 8 tertiary obstetric units in Ireland between May 2007 and October 2009. The primary aim of the ESPRiT study was to establish a threshold for birthweight discordance that serves as an independent predictor of adverse outcome in twin pregnancies. Institutional review board approval was obtained in each center, and participants gave written informed consent. Twin pregnancies with 2 viable fetuses that were identified from 11-22 weeks’ gestation were eligible for inclusion. The principal exclusion criteria were monoamnionicity and structural or chromosomal abnormalities in either twin. Cases were excluded subsequently if an intrauterine death of 1 or both fetuses occurred at <24 weeks’ gestation.
Study subjects underwent serial sonographic assessment of biometric parameters and multivessel Doppler ultrasound studies. Outcome data that were collected included maternal and obstetric characteristics, delivery and birthweight outcomes, and perinatal morbidity and mortality data.
Placental examination was carried out in the Pathology Department of the delivery hospital according to a defined study protocol. Formalin fixation of the placentas was carried out as per local practice in the delivery hospital. All placentas had chorionicity confirmed with gross and histologic examination of the intertwin membrane. Placental cord insertion site for each twin was recorded as central, marginal, or velamentous. Marginal cord insertion was defined as cord insertion at the edge of the placental disc. Velamentous insertion was defined as umbilical cord insertion into the membranes that are remote from the placental disc. All other cord insertions were defined as central .
All birthweights were recorded, and twins were recorded as SGA when their birthweight was <5th percentile for gestational age. Birthweight discordance was calculated as the absolute difference in birthweight between the twins and was expressed as a percentage of the weight of the larger twin. For the purposes of this analysis, significant birthweight discordance was defined as a difference in birthweight of >20%.
The overall distribution of cord insertion sites was compared between monochorionic and dichorionic twin pregnancies. The rate of each type of cord insertion was compared between twins that were SGA and those whose birthweight was appropriate for gestational age. To evaluate the association with birthweight, discordant twins were divided into 3 groups: the lighter twins of birthweight-discordant pairs, the heavier twins of birthweight-discordant pairs, and twins with birthweight discordance <20% (concordant). Prospective risk of SGA status and birthweight discordance with marginal and velamentous cord insertion was calculated.
Statistical analyses were performed with SAS software (version 9.1; SAS Institute Inc, Cary, NC). Relative frequencies were compared with the use of the chi-squared test. The paired Student t test was used to analyze continuous variables. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to estimate risks and the uncertainty around the risk estimates. A probability value of < .05 was considered statistically significant.
Results
One thousand twenty-eight twin pregnancies were recruited at 8 tertiary level centers in the ESPRiT study; 1001 patients completed the study and delivered at 1 of the 8 participating study centers. In 165 cases, the placentas were lost to follow up. In a further 20 cases, placental examination was completed, but the umbilical cord insertion site was not possible to determine or failed to be recorded. This left 816 twin pairs with data available for analysis. Table 1 outlines the clinical characteristics of this cohort: 20.2% of the pairs (n = 165) were monochorionic diamniotic, and 79.8% of the pairs (n = 651) were dichorionic. Monochorionic twins were delivered at a mean gestational age of 34.7 weeks, compared with 36.3 weeks’ gestation for dichorionic twins. Mean birthweight was significantly lighter in the monochorionic cohort when compared with the dichorionic cohort ( P = .0001); however, the proportion of SGA infants was similar in both groups ( P = .5). Birthweight discordance of ≥20% was present in 17.3% of twin pregnancies. There was no significant difference in the frequency of birthweight discordance between monochorionic and dichorionic twin pregnancies; 7.8% of the monochorionic twin pregnancies (n = 13) in this cohort were affected by TTTS.
| Variable | Monochorionic twins | Dichorionic twins | P value |
|---|---|---|---|
| Total, n (%) | 165 (20.2) | 651 (79.8) | |
| Mean birthweight, g a | 2207 ± 647 | 2517 ± 565 | .0001 |
| Mean gestational age at delivery, wk a | 34.7 ± 3.1 | 36.3 ± 2.4 | .0001 |
| Bodyweight discordance, n (%) | |||
| >20% | 32 (19.4) | 109 (16.7) | .42 |
| <5th percentile | 19 (5.8) | 91 (7) | .5 |
| Twin-twin transfusion syndrome, n (%) | 13 (7.8) |
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