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Prevalence and epidemiology
The incidence of twin pregnancies in the UK is 1 in 80 spontaneous conceptions, with higher-order multiples approximately following Hellin’s law. This states that the rate of n order of multiple pregnancy is 1 : 89n-1. The rate of monozygotic twinning is constant while the rate of dizygotic twinning is affected by family history, maternal age, the use of fertility drugs and treatment and geographical region, with the highest rate of 49 per 1000 maternities among the Yoruba of Nigeria.
The incidence of multiple pregnancies in the UK is increasing because of assisted reproduction technologies and increasing maternal age. About 25% of in-vitro fertilization pregnancies are multiple, compared to 1% for women who conceive naturally. In 2011 there were 16.1 multiple births per 1000 maternities, with women aged 45 and above having the highest multiple maternity rate of 99.3 per 1000. In that year, 11,330 women gave birth to twins, 172 to triplets and 3 to quadruplets or more. In 1976 there were 9.6 multiple maternities per 1000 maternities.1
Classification of multiple pregnancy
Twins result from the division of a zygote into two embryos or the fertilization and implantation of two different zygotes. Higher-order multiple pregnancies result from combinations of these two processes.
Zygosity refers to the number of zygotes involved in the twinning. Monozygotic twins result from the division of one zygote into two twins and are identical, having the same chromosomal composition. Dizygotic twins result from the fertilization and implantation of two eggs, resulting in non-identical twins that would have the same similarity as any other siblings. Twenty per cent of all twins are monozygotic.
Chorionicity denotes the number of placentas present and whether they are shared or not. Monochorionic twins share the same placenta and are invariably monozygotic, while dichorionic twins have separate placentas and could be mono- or dizygotic. Higher-order pregnancies could all have separate placentas, such as trichorionic triplets, or there may be combinations such as dichorionic triplets with one triplet having a separate placenta with the other two monochorionic.2
Amnionicity refers to the number of amniotic sacs and whether these sacs are shared by the fetuses. Monochorionic twins can be either monoamniotic or diamniotic, while dichorionic twins are always diamniotic. Depending on the stage at which it divides into two embryos, a monozygotic pregnancy could be dichorionic (< 4 days after fertilization and 30%) or monochorionic (70%). Monochorionic twins could be diamnionitic (4–8 days post-fertilization), monoamniotic (8–13 days) or conjoined (> 13 days).
Complications
Maternal complications of multiple pregnancy are usually secondary to the increased placental mass, while fetal complications depend on the fetal number and the type of multiple pregnancy.
Maternal complications include morning sickness, miscarriage, hyperemesis, anaemia, discomfort, gestational diabetes, gestational hypertension, pre-eclampsia, placenta praevia, polyhydramnios, preterm delivery, antepartum haemorrhage and postpartum haemorrhage (Table 3.1).
Exaggerated symptoms of pregnancy, e.g. vomiting, discomfort
Hyperemesis
Miscarriage
Anaemia
Pre-eclampsia
Gestational diabetes mellitus
Placenta praevia
Polyhydramnios
Antepartum haemorrhage
Postpartum haemorrhage
Fetal complications include chromosomal and structural abnormalities, intrauterine growth restriction and fetal death (Table 3.2). These complications could be selective, presenting management dilemmas for both the clinician and the patient. The risk of aneuploidy is dependent on the zygosity. In dizygotic twinning, as each twin is distinct, then the pregnancy risk is double that of a singleton pregnancy. For monozygotic twins, as they develop from the same zygote and are genetically similar, the pregnancy-specific risk of aneuploidy is the same as for singleton pregnancies. Rarely there might be heterokaryotypic monozygotic twins with chromosomal discordance. This is usually accompanied by fetal anomalies.
| Complication | Incidence |
|---|---|
| Chromosomal abnormality | 0.45% |
| Structural abnormality | 3.5% |
| Growth restriction | 29% |
| Preterm delivery (twins) | 50% |
| Preterm delivery (triplets) | 90% |
| Twin-to-twin transfusion syndrome (TTTS) | 15% |
| Twin reversed arterial perfusion (TRAP) sequence | 1% |
| Perinatal mortality rate (twins) | 14.8 per 1000 |
| Perinatal mortality rate (triplets) | 51.8 per 1000 |
Discordant fetal anomalies occur in 85% of anomalous twin pregnancies. While concordant anomalies are rare in dichorionic pregnancies, they occur in 18% of anomalous monochorionic pregnancies.3 Management options are expectant care, selective feticide or termination of the entire pregnancy. Selective feticide could be performed because of the abnormality but may also be indicated for obstetric reasons to prevent such complications as pre-eclampsia, polyhydramnios and preterm delivery. These complications may result from such anomalies as anencephaly and lethal trisomies. Spontaneous death of an anomalous twin can result in the death of, or neurological injury to, the normal twin in a monochorionic pregnancy. In monochorionic twins, selective feticide must be preceded or performed by procedures that separate both fetal circulations in order to avoid risks to the surviving twin. These risks are discussed later under Monofetal death.
The risk of perinatal mortality is 3–6 times higher for twins (14.8 per 1000 live births) and about 9 times higher for triplets (51.8) when compared with that for singleton pregnancies. This risk is significantly higher in monochorionic (11.6%) than dichorionic (5%) pregnancies. Though most of this excess risk for monochorionic twins compared to dichorionic twins occurs before 24 weeks, with the suggestion that survival rates are similar after this gestation, there is still increased risk of intrauterine demise with structurally normal well-grown monochorionic twins with no TTTS after 32 weeks compared to dichorionic twins.4
Specific to monochorionic twin pregnancy is the complication of twin-to-twin transfusion syndrome (TTTS), twin reversed arterial perfusion (TRAP) sequence and cord entanglement for monoamniotic twins.
Twin-to-twin transfusion syndrome
TTTS is caused by an imbalance in the haemodynamic communication between the circulation of the twins at the placenta, resulting in an anaemic donor twin with oligohydramnios (stuck twin) and a polycythaemic recipient with polyhydramnios and cardiac overload. TTTS complicates 15% of monochorionic pregnancies and if left untreated leads to perinatal mortality in 90% of cases. There is a 50% risk of neurological impairment due to prematurity or the intrauterine demise of one twin.5
There are three types of communication between the two circulations at the placenta. Artery-to-artery (AAA) and vein-to-vein anastomoses (VVA) are superficial on the chorionic plate and mediate bidirectional flow across the placenta, with AAA thought to be protective against TTTS. Artery-to-vein anastomoses (AVA) are deep in the placenta and mediate unidirectional flow across the placenta, and an imbalance in this flow is thought to be pathophysiologic for TTTS.
TTTS usually manifests in the second trimester (rarely in the early third trimester) and can be staged using the Quintero staging system (Table 3.3). Prognosis is better in early TTTS (stages I and II) compared with advanced disease (stages III and IV). Though this staging is useful for consistent description of the complication and outcomes, it does not always denote a logical order of progression of disease. There is an 80% risk of perinatal mortality and 15–20% risk of brain injury in survivors.6
| Stage | Classification |
|---|---|
| I | There is a discrepancy in amniotic fluid volume, with oligohydramnios of a maximum vertical pocket (MVP) ≤ 2 cm in one sac and polyhydramnios in the other sac (MVP ≥ 8 cm). The bladder of the donor twin is visible and Doppler studies are normal. |
| II | The bladder of the donor twin is not visible (during length of examination, usually around 1 hour) but Doppler studies are not critically abnormal. |
| III | Doppler studies are critically abnormal in either twin and are characterized as abnormal or reversed end-diastolic velocities in the umbilical artery, reverse flow in the ductus venosus or pulsatile umbilical venous flow. |
| IV | Ascites, pericardial or pleural effusion, scalp oedema or overt hydrops present. |
| V | One or both babies are dead. |
Fetoscopic laser ablation of the intertwin anastomoses on the chorionic plate should be the preferred treatment of severe TTTS (stages II, III and IV) before 26 weeks, as demonstrated by the Eurofetus trial, rather than amnioreduction or septostomy, because laser ablation corrects the pathophysiologic cause of the disease. Laser ablation is usually for stages II and III, with the attendant risks of preterm pre-labour rupture of membranes, preterm delivery, vaginal bleeding, abruption, chorioamnionitis, limb ischaemia, bowel atresia, recurrence of TTTS and loss of one or both twins. With laser ablation there is a 30–50% risk of perinatal mortality and a 5–20% risk of long-term neurologic handicap, with a 50% chance of intact survival of both twins.5,6
Surveillance should continue after laser ablation because there is a 14% risk of persisting or recurring TTTS and reversed TTTS. The optimal timing for the delivery of treated TTTS pregnancies is uncertain, but most of the pregnancies in published trials and case reviews were delivered at 33–34 weeks.
Management options for severe TTTS after 26 weeks include delivery, with the attendant risks of prematurity and perinatal morbidity and mortality; amnioreduction, with attendant risks of preterm labour; and laser ablation. However, laser ablation becomes technically more difficult to perform with advancing gestation. Stage I TTTS may best be managed expectantly, because 75% of cases remain stable or regress and survival rates exceed 80%.
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