This is the first edition of this guideline, published in December 2008.
The purpose of this guideline is to describe and, if possible, quantify the problems associated with monochorionic placentation and to identify the best evidence to guide clinical care, including routine fetal surveillance and treatment of complications at secondary and tertiary levels.
A monochorionic twin pregnancy is one in which both babies are dependent on a single, shared placenta. Around one-third of twin pregnancies in the UK have monochorionic placentas. Monochorionic placentation can also occur in higher-order multiples. There has been a recent increase in all types of multiple pregnancies with the increasing use of assisted reproductive techniques. Monochorionic (MC) and dichorionic (DC) twin pregnancies share increased risks of preterm birth, fetal growth restriction, pre-eclampsia, maternal pregnancy symptoms and postpartum haemorrhage. The particular challenges of monochorionic pregnancies arise from the vascular placental anastomoses that are almost universal and connect the umbilical circulations of both twins: twin–twin transfusion syndrome (TTTS), the consequences to the co-twin of fetal death and the management of discordant malformations. In addition, monochorionic, monoamniotic pregnancies (1% of twin pregnancies) carry a very high risk of cord entanglement. TTTS complicates 10–15% of MC pregnancies; the placentas are more likely to have unidirectional artery–vein anastomoses and less likely to have bidirectional artery–artery anastomoses. A number of features of TTTS still elude full understanding; discordant activation of the renin–angiotensin system may be important. In a series of 82 consecutive MC pregnancies with blinded placental injection studies, there were similar numbers of artery–vein and vein–vein anastomoses in TTTS when compared with non-TTTS pregnancies. Fetal survival was better if artery–artery anastomoses were demonstrated after delivery. TTTS is found in monochorionic, monoamniotic (MCMA), as well as monochorionic, diamniotic (MCDA) pregnancies. TTTS is more common in MCDA pregnancies than MCMA pregnancies, possibly reflecting that there are more protective artery–artery anastomoses in the latter. Rarely (in approximately 5% of cases), the transfusion may reverse during pregnancy, with the donor fetus demonstrating features of a recipient fetus and vice versa. The pathophysiological basis for this is unclear. Reverse transfusion can occur following laser surgery to the placenta. Unequal placental sharing and peripheral, ‘velamentous’ cord insertions are common in TTTS. Significant intrauterine size discordance occurs in MC twins in the absence of TTTS in approximately 10% of pregnancies. The incidence of size discordance is as great in DC pregnancies in some series but management of discordant growth restriction may be more difficult in MC pregnancies. Discordant fetal growth restriction may be differentiated from TTTS by the absence of polyhydramnios in one of the amniotic sacs, although the small twin may have oligohydramnios owing to placental insufficiency.
This guideline can be downloaded as a pdf:
