Selective intrauterine growth restriction in monochorionic twins: pathophysiology, diagnostic approach and management dilemmas

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Summary

Selective intrauterine growth restriction (sIUGR) in monochorionic twins is associated with a substantial increase in perinatal mortality and morbidity for both twins. Clinical evolution depends on the combination of the effects of placental insufficiency in the IUGR twin with inter-twin blood transfer through placental anastomoses. Classification of sIUGR into types according to the characteristics of umbilical artery diastolic flow in the IUGR twin permits the differentiation of clinical and prognostic groups. sIUGR type I has normal diastolic flow and relatively good outcome. Type II is defined by persistently absent/reverse end-diastolic flow and is associated with a high risk of intrauterine demise of the IUGR twin and/or very preterm delivery. Type III is defined by the presence of intermittent absent/reverse end-diastolic flow (iAREDF), and is associated with 10–20% risk of unexpected fetal demise of the smaller twin and 10–20% risk of neurological injury in the larger twin. The management strategy for sIUGR with abnormal umbilical artery Doppler (types II and III) remains a challenge, and may include elective fetal therapy or close surveillance with fetal therapy or elective delivery in the presence of severe fetal deterioration. Small clinical series reporting the use of cord occlusion or laser therapy in severe cases suggest that the outcome of the larger twin might be improved. There is probably no single optimal strategy, since decisions will ultimately be influenced by the severity of IUGR, gestational age, parents’ wishes and technical issues.

Introduction

Selective intrauterine growth restriction (sIUGR) is a common condition associated with monochorionic (MC) pregnancy. It is increasingly considered to be an important complication of MC twins, with potentially significant risks of intrauterine fetal demise (IUFD) or neurological adverse outcome for both twins.1, 2, 3, 4, 5 Introduction of skilled sonographic evaluation, a better understanding of different Doppler patterns and of the anatomy of MC placenta, together with the development of fetoscopic techniques have all contributed to a deeper scientific understanding of this condition. This review summarizes the unique pathophysiology of sIUGR, proposes a classification system which may facilitate to interpret the apparently wide clinical variability observed in this condition, and suggests management approaches according to this classification.

Section snippets

Definition and prevalence of sIUGR

The term ‘selective intrauterine growth restriction’ in monochorionic pregnancies is applicable to cases where the estimated fetal weight (EFW) of the small fetus falls below the 10th percentile. Significant fetal weight discordance is an important element of the clinical picture, which will often accompany this condition, but is not necessary for diagnosis. This is defined by different authors as discordance between the EFW of two fetuses >25%,6, 7 and is calculated as the difference between

Aspects of the pathophysiology and natural history of sIUGR in MC twins

In recent years the pathophysiological insight of sIUGR has been substantially improved, although the ability to reliably predict the clinical outcome remains elusive – this will probably remain the case, as the clinical presentation and outcome seem to depend on a combination of multiple factors. Until a few years ago the most feared complication of sIUGR in MC twins was the intrauterine demise of the growth-restricted twin, which carries an associated postmortem risk of acute feto-fetal

Classification of different types of sIUGR on the basis of umbilical artery Doppler: rationale and clinical use

As stated above, the clinical evolution of sIUGR is subject to remarkable variability, and the identification of groups with similar clinical behavior may substantially facilitate clinical management. To date, the clinical technique that best achieves this goal is umbilical artery (UA) Doppler of the IUGR twin. In singleton and DC twin pregnancies, UA Doppler is a key parameter for the diagnosis and surveillance of fetuses with IUGR secondary to placental insufficiency.23 However, in MC twins

Definition and placental features

The type I Doppler pattern is distinguished by positive diastolic flow in the umbilical artery of the small twin. Placental anastomotic patterns in type I pregnancies are similar to uncomplicated monochorionic pregnancies, resulting in a fair number of anastomoses and bidirectional fetal flow interchange. Such interchange favours blood from the larger twin working in a compensatory manner since, even if marginally, it is better oxygenated, and this attenuates the effects of placental

Definition and placental features

Type II pattern is characterized by persistently AREDF in the UA. As in type I, sIUGR type II pregnancies show a distribution of placental anastomoses quite similar to uncomplicated MC twins, but with a more severe placental discordance.18 Fetal territory of the IUGR twin is usually extremely small in type II pregnancies, but again the fetal weight/placental discordance ratio is significantly lower than in uncomplicated MC twins,18 illustrating how inter-twin blood transfusion attenuates the

Characteristic Doppler findings and placental features

Type III sIUGR is defined by the presence of iAREDF in the UA Doppler of the IUGR twin. The characteristic feature of this Doppler pattern, unique to monochorionic twins, is the alternation of phases of positive with phases of absent/reverse diastolic flow, normally but not always in a cyclical fashion (Figure 1). The observation of this sign indicates the presence of a large placental AA anastomosis,22, 26 which facilitates transmission of the systolic waveforms of one twin into the umbilical

Cardiac consequences of selective IUGR

The particular hemodynamic situation created by the existence of vascular anastomoses between two fetuses with discordant weights may entail cardiovascular consequences for the larger twin. This situation is most evident in type III pregnancies, where hypertrophic cardiomyopathy-like changes are observed in the normal twin in a substantial proportion.35 Thus, an enlarged heart in the normally grown fetus should be regarded as part of the normal spectrum of findings in type III pregnancies.

Late-onset SIUGR

A recent study has described the clinical outcome and placental characteristics of late-onset sIUGR. This was defined as birth weight discordance of ≥25% first diagnosed after 26 weeks of gestation and thus not observed at 20 weeks.5 The rate of late-onset sIUGR was reported to be 6.3%, 13 in a cohort of 208 MC pregnancies. Clinically, it was characterized by a benign course with almost concordant growth until the late second trimester, with discordance developing progressively from then onwards

Conflict of interest statement

None declared.

Funding sources

E.E. and F.C. are supported by Rio Hortega Research fellowships from the Spanish Fondo de Investigaciones Sanitarias.

Practice points

  • MC twin gestations complicated by sIUGR are at high risk of perinatal complications and should be managed in dedicated high risk pregnancy units.

  • Clinical evolution of early-onset sIUGR is a function of placental sharing discordance and inter-twin blood flow determined by the anastomotic pattern, and this may lead to substantial clinical differences in apparently

References (35)

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