The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 4

CHORIONICITY AND PREGNANCY COMPLICATIONS

Miscarriage

In singleton pregnancies with a live fetus demonstrated at the 11-13+6 weeks scan the rate of subsequent miscarriage or fetal death before 24 weeks is about 1%. The rate of fetal loss in dichorionic twins is about 2% and in monochorionic twins it is about 10%. This high mortality confined to monochorionic pregnancies is the consequence of severe early-onset TTTS. Reduction of the excess fetal loss in twins, compared to singletons, can only be achieved through early identification of monochorionic pregnancies by ultrasound examination at 11-13+6 weeks, close surveillance and appropriate treatment, with endoscopic laser coagulation of the communicating placental vessels, in those that develop severe TTTS.

Perinatal mortality

The perinatal mortality rate in twins is about 5-times higher than in singletons. This increased mortality, which is mainly due to prematurity-related complications, is higher in monochorionic (5%) than dichorionic (2%) twin pregnancies. In monochorionic twins, an additional complication to prematurity is TTTS.

Early preterm delivery

The most important complication of any pregnancy is delivery before term and especially before 32 weeks. Almost all babies born before 24 weeks die and almost all born after 32 weeks survive. Delivery between 24 and 32 weeks is associated with a high chance of neonatal death and handicap in the survivors. The chance of spontaneous delivery between 24 and 32 weeks is about 1% in singletons, 5% in dichorionic and 10% in monochorionic twin pregnancies.

Growth restriction

In singleton pregnancies the prevalence of babies with birth weight below the 5th centile is 5%, in dichorionic twins it is about 20% and in monochorionic twins it is 30%. Furthermore, the chance of growth restriction of both twins is about 2% in dichorionic and 8% in monochorionic pregnancies.

In singleton pregnancies the two main factors determining fetal growth are genetic potential and placental function. In monochorionic twin pregnancies both factors should be the same for the two fetuses. Consequently, inter-twin disparities in growth are likely to reflect the degree of unequal splitting of the initial single cell mass or the magnitude of imbalance in the bidirectional flow of fetal blood through placental vascular communications between the two circulations. In contrast, since about 90% of dichorionic pregnancies are dizygotic, inter-twin disparities in size would be due to differences in genetic constitution of the fetuses and their placentas.

Pre-eclampsia

The prevalence of pre-eclampsia is about 4-times greater in twin than in singleton pregnancies but it is not significantly different between monochorionic and dichorionic twins.

Death of one fetus

Intrauterine death of a fetus in a twin pregnancy may be associated with adverse outcome for the co-twin but the type and degree of risk is dependent on the chorionicity of the pregnancy. In singleton pregnancies death and retention of the fetus may be associated with maternal disseminated intravascular coagulation; however, in twin pregnancies with one dead fetus this complication has only rarely been reported.

Death of one fetus in dichorionic pregnancies carries a risk of death or handicap of 5-10% to the remaining fetus. This is mainly due to preterm delivery, which may be the consequence of release of cytokines and prostaglandins by the resorbing dead placenta. In monochorionic twins, there is at least a 30% risk of death or neurological handicap to the co-twin because in addition to preterm delivery, there is a risk of acute hypotensive episodes due to hemorrhage from the live fetus into the dead fetoplacental unit. Intrauterine blood transfusion within 24 hours of death of the co-twin may prevent fetal death.

Structural defects

Structural fetal defects in twin pregnancies can be grouped into those which also occur in singletons and those specific to the twinning process; the latter being unique to monozygotic twins. For any given defect the pregnancy may be concordant or discordant in terms of both the presence or type of abnormality and its severity. The prevalence of structural defects per fetus in dizygotic twins is the same as in singletons, whereas the rate in monozygotic twins is 2-3 times higher. Concordance of defects (both fetuses being affected) is uncommon, being found in about 10% of dichorionic and 20% of monochorionic pregnancies.

In monozygotic pregnancies possible causes of discordancies for defects include asymmetric splitting of the cell mass, resulting in unequal potential for development (the ‘Christmas cracker’ hypothesis) or splitting after laterality gradients are determined, resulting in malformations of laterality, such as cardiac and midline defects.

Multiple pregnancies discordant for a fetal abnormality can essentially be managed expectantly or by selective fetocide of the abnormal twin. In cases where the abnormality is non-lethal but may well result in serious handicap the parents need to decide whether the potential burden of a handicapped child is enough to risk the loss of the normal twin from fetocide-related complications. In cases where the abnormality is lethal it may be best to avoid such risk to the normal fetus, unless the condition itself threatens the survival of the normal twin. In the case of dichorionic pregnancies fetocide can be carried out by intracardiac injection of potassium chloride, whereas in monochorionic pregnancies fetocide necessitates occlusion of the umbilical cord vessels.

Twin-to-twin transfusion syndrome

In monochorionic twin pregnancies there are placental vascular anastamoses which allow communication of the two fetoplacental circulations; these anastomoses may be arterio-arterial, veno-venous, or arterio-venous in nature. Anatomical studies demonstrated that arterio-venous anastomoses are deep in the placenta but almost always proceed through the cotyledonary capillary bed. In about 30% of monochorionic twin pregnancies, imbalance in the net flow of blood across the placental vascular arterio-venous communications from one fetus, the donor, to the other, the recipient, results in twin-to-twin transfusion syndrome; in about half of these the condition is severe (Figure 9).

Severe disease, with the development of polyhydramnios, becomes apparent at 16-24 weeks of pregnancy. The pathognomonic features of severe TTTS by ultrasonographic examination are the presence of a large bladder in the polyuric recipient fetus in the polyhydramniotic sac and ‘absent’ bladder in the anuric donor that is found ‘stuck’ and immobile at an edge of the placenta or the uterine wall where it is held fixed by the collapsed membranes of the anhydramniotic sac (Figure 3).

Figure 3. Severe twin-to-twin transfusion syndrome at 20 weeks of gestation. In the polyuric recipient there is a large bladder and polyhydramnios (left) and the anuric donor is held fixed to the placenta by the collapsed membranes of the anhydramniotic sac (right).

Early prediction of twin-to twin transfusion syndrome

Ultrasonographic features of the underlying hemodynamic changes in severe TTTS may be present from as early as 10-13+6 weeks of gestation and manifest as increased nuchal translucency (NT) thickness in one or both of the fetuses. In monochorionic twin pregnancies at 11-13+6, the presence of increased NT thickness in one of the fetuses increases the risk for subsequent development of TTTS by a factor of three. Another early ultrasound marker of severe TTTS may be abnormal Doppler flow velocity waveforms in the ductus of the recipient twin. Intertwin discrepancies in crown-rump length do not appear to be predictive of subsequent development of TTTS.

An early manifestation of disparity in amniotic fluid volume due to TTTS is inter-twin membrane folding (Figure 4). In about 30% of monochorionic twin pregnancies at 15-17 weeks of gestation there is a membrane folding, and in about half of such cases there is progression to the polyhydramnios / anhydramnios sequence of severe TTTS; in the other half there is moderate TTTS with large discrepancies in amniotic fluid volume and fetal size persisting throughout pregnancy. In two-thirds of monochorionic twin pregnancies, there is no membrane folding and these pregnancies are not at increased risk for miscarriage or perinatal death due to TTTS.

Twin reversed arterial perfusion sequence

The most extreme manifestation of twin-to-twin transfusion syndrome, found in approximately 1% of monozygotic twin pregnancies, is acardiac twinning. This twin disorder has been named twin reversed arterial perfusion (TRAP) sequence because the underlying mechanism is thought to be disruption of normal vascular perfusion and development of the recipient twin due to an umbilical arterial-to-arterial anastomosis with the donor or pump-twin. At least 50% of donor twins die due to congestive heart failure or severe preterm delivery, due to polyhydramnios. All perfused twins die due to the associated multiple malformations. Prenatal treatment is by occlusion of the blood flow to the acardiac twin by ultrasound-guided diathermy of the umbilical cord or laser coagulation of the umbilical cord vessels within the abdomen of the acardiac twin, which is carried out at about 16 weeks of gestation.

Importance of prenatal determination of chorionicity

• Chorionicity, rather than zygosity, is the main factor determining pregnancy outcome.
• In monochorionic twins the rates of miscarriage, perinatal death, preterm delivery, fetal growth restriction and fetal abnormalities are much higher than in dichorionic twins.
• Death of a monochorionic fetus is associated with a high chance of sudden death or severe neurological impairment in the co-twin.