Fetal Complications in Monochorionic Twin Pregnancies
Monochorionic twins can be discordant in amniotic fluid volume, in size, in the presence of congenital abnormalities and in hemoglobin concentration. Severe amniotic fluid discordance is typical for TTTS, which complicates 8–10% of monochorionic twin gestations, usually between 16 and 26 weeks TTTS is the most important cause of death and handicap in monochorionic twin pregnancies Its sonographic diagnosis is based on strict criteria of amniotic fluid discordance. Most European centers adhere to gestational-age-dependent criteria to define polyhydramnios in the recipient sac: deepest vertical pocket > 8 cm prior to 20 weeks > 10 cm after 20 week. In contrast, in the United States, the 8-cm cutoff is used more often throughout gestation . Both continents agree on the definition of oligohydramnios in the donor’s sac ( < 2 cm deepest vertical pocket).
TTTS is thought to arise from an intertwin transfusion imbalance across the vascular anastomoses with hypervolemia, polyuria and polyhydramnios in the recipient and hypovolemia, oliguria and oligo-anhydramnios in the donor. However, since most do not have significant intertwin hemoglobin discordance , hormonal factors almost certainly play an important role as well . The presentation of TTTS is variable and its course mainly unpredictable. As such, TTTS may present as a slowonset disease or be rapidly progressive. Most likely, TTTS encompasses different diseases with a variable contribution of transfusion imbalance, unequal placental sharing and hormonal dysfunction.
Untreated mid-trimester TTTS carries an extremely poor prognosis. Fetoscopic laser coagulation of the vascular anastomoses is the only curative and currently the best available treatment . Compared to amniodrainage, laser coagulation results in better survivalrates and in a reduced neurological morbidity . Nevertheless, survival rates after fetoscopic laser surgery are only about 70% , and long-term developmental delay occurs in about 15% of survivors .
Fetal loss is attributable to intrauterine demise of one twin, which most commonly occurs within the 1st postoperative week and is probably related to cardiac failure, unequal placental sharing or incomplete separation. Miscarriage and preterm birth (often after ruptured membranes) are other important causes of fetal loss . Developmental delay may be related to the disease itself, laser surgery and most importantly to preterm birth .
The true mortality and morbidity of TTTS are likely to be still higher, because a not insignificant number of patients present too late for treatment with demise of one or both twins, with ruptured membranes or in active labor, and are not included in the outcome series on fetoscopic laser surgery. In our observational study of monochorionic twins followed from the first trimester, TTTS had a mortality of 55%, which is higher than the 30% usually quoted in fetoscopic laser series. Future research aims to improve survival rates by better prediction, a better case selection for surgery as well as better surgical techniques to reduce the failure rate and risk of postoperative membrane rupture.
Monochorionic twins can also be discordant in size. Isolated discordant growth is usually defined as an intertwin size difference of 25% or more in the absence of TTTS. Usually, in these cases, there is a certain degree of amniotic f luid discordance, too. Because of its smaller size, the smaller twin will necessarily produce less urine than its larger co-twin. Also, similar to singletons that are growth restricted, in severe cases, the smaller twin may develop oligo- or anhydramnios with deepest pockets < 2 cm. Although discordant growth is commonly present in TTTS, the distinction between isolated discordant growth and TTTS is easily made by the absence of polyhydramnios in the appropriately grown twin as defined by the TTTS criteria described above. Isolated discordant growth complicates about 15% of monochorionic twin pregnancies. However, in contrast to TTTS, it has a much lower mortality, about 10 versus 55% . Discor-dant growth seems also much less likely to cause developmental delay compared to TTTS . Nevertheless, as long as the twins are undelivered and especially prior to 26 weeks, about 20% of pairs with discordant growth will progress to TTTS. In fact, the diagnosis of discordant growth can only be made in retrospect when the twin pregnancy comes to an end: either at birth or in the event of intrauterine demise of one or both twins. At present, there are no good markers to differentiate the pregnancies that will remain simply growth discordant from those that will progress to TTTS. Therefore, careful surveillance remains indicated in all pregnancies with discordant growth.
There seem to be two patterns of isolated discordant growth in monochorionic twins. One is early in onset and presents at or before 20 weeks. The other is late in onset and presents after 26 weeks but is usually diagnosed only at the time of birth. As expected, early-onset discordant growth carries the highest mortality (about 15%) and often there is an abnormal f low pattern in the umbilical artery of the smaller twin. Placentas of twin pairs with early-onset discordant growth are typically unequally shared with an elaborate intertwin circulation and a large artery-to-artery anastomosis. On the other hand, the mortality of late-onset discordant growth is much lower (about 4%) and usually umbilical-artery Doppler scans are normal throughout pregnancy. In one third of pregnancies with late-onset discordant growth, a late intertwin transfusion imbalance underlies the growth discordance (TAPS), which typically presents with severe intertwin hemoglobin discordances. These placentas are usually equally shared with only a few tiny and mostly unidirectional anastomoses.
Next to this classification based on time of presentation, growth-discordant monochorionic twin pregnancies can also be grouped according to the flow pattern in the umbilical artery. In monochorionic twin pregnancies, umbilical-artery Doppler waveforms reflect not only downstream placental vascular resistance but also blood flow across the anastomoses. If umbilical-artery Doppler patterns are normal (type I), outcome is generally good. In these cases, placental sharing is mildly unequal and large artery-to-artery anastomoses are usually present. Less often, end-diastolic f low may be persistently absent or reversed (type II). These cases behavesimilar to growth-restricted singleton pregnancies, and an absent or reversed a-wave of the ductus venosus usually indicates imminent fetal demise. Type II placentas are more unequally shared than type I placentas, but typical lylack the large artery-to-artery anastomoses. Type II cases have the worst prognosis as 90% will eventually show signs of deterioration and imminent demise. Finally, in type III, which is most common in early-onset discordant growth, end-diastolic flow is intermittently absent or reversed in the umbilical artery of the smaller twin because of a large artery-to-artery anastomosis.Type III cases have an i ntermediate prognosis, but are themost unpredictable. Placental sharing is grossly unequal and nearly all have large artery-to-artery anastomoses. Because of these large artery-to-artery anastomoses, unexpected demise without any signs of deterioration occurs in about 15% and half of these are double demises.
The best management of discordant growth in monochorionic twin pregnancies is not yet well established. In our center, we follow cases with early-onset discordant growth with weekly ultrasound scan to detect any progression to TTTS or signs of deterioration. From 28 weeks onward and for types II and III, we monitor daily and deliver electively 32–33 weeks after the administration of steroids. We offer laser coagulation of the vascular anastomoses in case of progression to TTTS. On the other hand, we offer selective feticide of the smaller twin by umbilical cord coagulation in type II and III cases if there are complicating features prior to viability, e.g. when the smaller twin develops anhydramnios, persistently absent or reversed flow in the ductus venosus or an arrest of growth over a 3-week period. With this policy of careful surveillance, elective preterm birth and selective feticide only for complicated cases, the survival rate for isolated early-onset discordant growth is 85%. The role of laser coagulation in the management of isolated discordant growth is still controversial. Unequal placental sharing is frequently present, especial lyin type III cases, and here laser separation may leave too little placental territory for the smaller twin to allow an intact survival. On the other hand, separation will protect the larger twin better in the event of spontaneous demise of its growth-restricted co-twin. So far, small case series have failed to demonstrate a benefit of laser separation in these cases. For late-onset discordant growth, surveillance of the middle cerebral artery peak systolic velocity (MCA-PSV) may play an important role to detect cases with TAPS . Future research needs to address better predictors to discriminate between discordant-growth twins with a favorable outcome and those at high risk of TTTS, intrauterine demise or long-term neurological morbidity who may benefit from invasive antenatal therapy.
Monochorionic twin pregnancies can also be discordant in the presence of congenital abnormalities, which are more common in monochorionic twins, probably due to a teratogenic effect of embryo cleavage or because of complications of the shared circulation. As such, major congenital defects are found in about 6% of pregnancies and usually only one twin is affected. Cardiac anomalies are especially prevalent amongst monochorionic twins. Therefore, all monochorionic twins should benefit from detailed sonographic follow-up by experienc