Apart from the general risks associated with twin pregnancies, monochorionic (MC) pregnancies pose specific risks to unborn fetuses:
1. Twin pregnancy with one spontaneous reduction/ intrauterine fetal demise
- Sometimes for twins, one twin has spontaneous miscarriage while the other twin survives. This is called ‘spontaneous reduction’ to singleton. The twin which remains viable is called the ‘surviving twin’. If it happens in very early gestation, the small embryo which has succumbed may be completely reabsorbed or incorporated into the placental membranes, and it is called the ‘vanished twin’.
- If this happens after 24 weeks, it is called ‘twin pregnancy with one intrauterine fetal demise’. The succumbed fetus would still be seen on ultrasound.
- For MC twins, the vessels of the two placentae are connected. Therefore, there is a concern that blood might be shunted to the dead twin, causing ischaemic attack of the surviving twin. The chance of death or neurological damage of the surviving twin is estimated to be ~25%. This risk should be much lower if it happens in the first trimester before 9 weeks of gestation.
- Serial ultrasound is advised to look for signs of brain damage and to monitor the growth of the surviving twin.
2. Twin-twin transfusion syndrome (TTTS)
- As the vasculature of the placentae are connected, the blood distribution to the fetuses might not be even. TTTS happens if there is an unbalanced unidirectional blood flow through the placental vessels (typically arterial to venous connections). The reported incidence is 15% among MC twin pregnancies.
- The recipient twin (one which receives surplus of blood) may have excessive amniotic fluid volume (called ‘polyhydramnios’), high red blood cells (called ‘polycythaemia’) and be bigger in size. Some may develop heart failure and generalised oedema (called ‘hydrops’).
- The donor twin (one which does not receive enough blood as the placenta ‘donates’ the blood to the other twin) will have little amniotic fluid volume (called ‘oligohydramnios’), low red blood cells (called ‘anaemia’) and fetal growth restriction. Sometimes this donor twin might be stuck to one corner of the uterus when there is no amniotic fluid within the amniotic sac.
- TTTs comes in different degrees of severity. In most severe cases, the mortality rates for the twins approach 100% if un-treated.
- TTTs can develop at any period of the pregnancy. The severe one happens early, between 14 and 26 weeks gestation. It is therefore important to have serial ultrasound assessment, at 2-4 weeks’ intervals in the second trimester, to look for TTTS for monochorionic diamniotic twin pregnancies.
- For severe TTTS, antenatal treatment is necessary. Currently, the best treatment is fetoscopic laser photocoagulation. It is a surgery performed to separate the vascular connection at the surface of the two joint placentae by laser. The rate of survival of at least one fetus following the procedure is ~75%. The chance of survival of both twins is ~40%. The operation carries small risks of miscarriage, rupture of membranes and separation of the placental attachment (placental abruption). Therefore, it is only performed if indicated.
3. Twin anaemia-polycythaemia sequence (TAPS)
- This is an atypical chronic form of TTTS. In contract to the classical TTTS (see above), TAPS is caused by imbalanced transfusion in a few small sized artery-to-vein vascular connections. These vessels are less than 1mm in diameter.
- The donor twin will have anaemia while the recipient twin will have polycythaemia. The amniotic fluid volumes of both twins are normal.
- It can occur following fetoscopic laser photocoagulation for treatment of TTTS. Less commonly, it may also happen spontaneously.
- The diagnosis is by doppler ultrasound of an artery of fetal brain (middle cerebral artery. MCA). The peak blood flow velocity (PSV) is measured. TAPS is diagnosed if fetal anaemia (MCA-PSV > 1.5 multiples of median (MoM)) in one twin while polycythaemia occurs in the other one (MCA-PSV <0.8 MoM).
- At delivery, the anaemic twin looks pale while the polycythaemic one look dark. The difference of the haemoglobin between the twins will be ≥ 8.0 g/dL.
- Treatment include close monitoring if mild, delivery if advanced gestation, and intrauterine blood transfusion or fetoscopic laser photocoagulation if delivery is not an option.
4. Selective intrauterine growth restriction (sIUGR)
- This refers intrauterine growth restriction affecting one of the twins due to unequal sharing of the placenta.
- In the case of MC placenta, each twin has a share of the placental territory. Sometimes, there is an unequal share and the twin with sIUGR can only ‘own’ a small portion of the placenta.
- It affects 10-15% of monochorionic twin pregnancies.
- The twin with severe sIUGR may succumb leading to ischaemic attack and hence neurological damage of the surviving twin.
- If this is detected in early or mid-trimester, the treatment options include selective fetocide or fetoscopic laser photocoagulation. If it occurs in the third trimester, delivery might be considered.
5. Monochorionic monoamniotic (MCMA) twins
- MCMA twins are rare, accounting for ~1% of all twin pregnancies. The two twins are not separated by an amniotic membrane and hence there is a high chance of cord entanglement. Cord compression as a result of entanglement may reduce or even cut off the blood supply to the fetuses, leading to fetal compromise. MCMA twins are also at risk of TTTS and preterm birth. High perinatal mortality (30-70%) has been reported for this type of twins. There is no consensus on the management of MCMA twins but many units have serial ultrasound to search for cord entanglement. If cord entanglement is noted, very intensive monitoring using doppler ultrasound and fetal cardiotocogram is commonly carried out. Many advocate to have delivery by elective caesarean section at 34 weeks for this type of twin pregnancies.
- Conjoint twins’ is an extremely rare condition, which only occurs in MCMA type of twins. Antenatal diagnosis could be made in the first trimester by ultrasound.