Ultrasound is the primary tool for monitoring growth in multiple pregnancies for two reasons. First, they are at high risk of intrauterine growth restriction (IUGR), with 25% of twins being small for gestational age at birth. In most cases IUGR will be discordant affecting one twin only. Second, abdominal palpation and symphysis-fundal height measurement are unreliable as indices of individual fetal growth as, instead, they reflect total intrauterine growth.
There is no agreement on the ideal frequency of ultrasound examinations in twins, but a conservative policy for detecting IUGR in DC twins is four weekly scanning from 24 weeks, with further scans and/or Doppler measurements as indicated. MC twins should be scanned at fortnightly intervals from 16 weeks, both to allow early diagnosis and thus treatment of twin-twin transfusion syndrome, and to pre-empt intrauterine death from IUGR through timely delivery (or pre-emptive cord occlusion prior to viability) to protect the healthy co-twin.
There is controversy as to whether singleton or twin bio-metric charts should be used. The former seems the more logical, as twins are at high risk of IUGR with attendant morbidity, and separate charts are not used for other high-risk groups, such as pre-eclamptics or diabetics. Further, increasing emphasis is placed on growth profile and fetal condition (liquor volume, umbilical Doppler). Many use percentage discordancy in estimated fetal weight (=100* [EFWlarger — EFWsmaller] EFWlarger) as an index of discordant IUGR. Discordancy >25-30% has some predictive value in MC twins for poor outcome in twin-twin transfusion syndrome (TTTS) and for fetal death , whereas in DC twins, it is controversial whether or not it denotes poorer outcome independent of the degree of IUGR [27,28].
The standard principle of management in IUGR (i.e. deliver when the risks of continued intrauterine outweigh those of extra-uterine existence) needs modification in twin pregnancy to account for the risks to both fetuses. Thus whereas cessation in fetal growth with preterminal Doppler studies might warrant delivery at 25 weeks with a singleton fetus, discordant IUGR with this picture in DC twins might better be managed by allowing the IUGR fetus to die in utero, sparing the healthy fetus the risks of iatrogenic prematurity.
In MC twins the decision-making is more complex. On the one hand, latency (absent end diastolic frequencies in the umbilical artery) may persist weeks longer without decompensation than in DC twins . On the other hand, delivery or pre-emptive cord occlusion must be instituted before any intrauterine death to protect the co-twin from acute transfusional sequelae. Such balancing of risks is always difficult and decision making should occur in concert with the parents and neonatal paediatricians.
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