Abstract
Cerebral palsy is nearly three times more common and perinatal mortality is four times higher in twins compared with singleton pregnancies. Perinatal mortality is two to three times higher in monochorionic twins compared with dichorionic twins due to the presence of placental vascular anastomoses. Monochorionic diamniotic twins carry a 25% risk of twin-to-twin transfusion syndrome (TTTS) [1].
Key Facts
Zygosity in twins: 80% are dizygotic and 20% are monozygotic.
Dizygotic twins arise from fertilization of two ova and monozygotic twins arise from one ovum. Therefore all dizygotic twins have separate placentas and separate chorionic amniotic sacs.
Monozygotic twins can divide into two at different stages, resulting in three types:
1.Dichorionic diamniotic (DCDA): Twins have separate placentas and chorionic and amniotic sacs. Approximately 30% of monozygotic twins are DCDA and they are of the same sex.
2.Monochorionic diamniotic (MCDA): Each twin develops in its own amniotic cavity but the placenta and chorion are shared. Approximately 70% of monozygotic twins are MCDA.
3.Monochorionic monoamniotic (MCMA): The placenta and chorionic and amniotic cavities are shared. Fewer than 1% of monozygotic twins are MCMA.
Incidence Currently around 15 per 1000 maternities. The incidence has increased by 50% in developed countries over the past two decades. The monozygous twin rate is more constant at 4 per 1000 maternities and the dizygous twin rate is more variable depending on the population and prevalence.
Key Implications
Cerebral palsy is nearly three times more common and perinatal mortality is four times higher in twins compared with singleton pregnancies. Perinatal mortality is two to three times higher in monochorionic twins compared with dichorionic twins due to the presence of placental vascular anastomoses. Monochorionic diamniotic twins carry a 25% risk of twin-to-twin transfusion syndrome (TTTS) [1]. The main reasons for high perinatal mortality in twin pregnancy are
Prematurity
Intrauterine growth restriction
Congenital malformations
Complications due to placental vascular connections
Umbilical cord prolapse
Maternal morbidity is high due to
Hyperemesis gravidarum
Antepartum haemorrhage
Anaemia
Preeclampsia
Placental abruption
Placenta praevia
Postpartum haemorrhage
Complications of internal podalic version and breech extraction of the second twin include
Skeletal injury (femur, humerus, hips, skull)
Visceral injury (kidney, liver, spleen)
Neural injury (facial nerve palsy, hypoxic cerebral damage)
Cord prolapse
Hand prolapse
Maternal
Uterine rupture
Placental abruption
Vaginal and perineal trauma
Postpartum haemorrhage
Puerperal sepsis
Key Pointers
The dizygotic twin rate has increased by about 50% over the past two decades due to the wide use of ovarian stimulation regimes in the treatment of subfertility.
Exaggerated symptoms in early pregnancy with symphysio-fundal height greater than the period of amenorrhoea or the presence of multiple fetal parts should arouse the suspicion of multiple pregnancy in a low-resource setting where recourse to a routine ultrasound scan in early pregnancy is not available.
Key Diagnostic Signs
Diagnosis can be made by ultrasound in the first trimester and the chorionicity has to be assessed before 14 weeks [2].
Inspect the placental base of the membrane separating the two sacs.
Thick ‘lambda’ shape – dichorionic diamniotic twins
Thin ‘T’ shape – monochorionic diamniotic twins
Colour Doppler can be used to demonstrate a functional artery-to-artery anastomosis which provides definitive proof of monochorionicity. Doppler ultrasound is also useful in monitoring fetal compromise in twins in late pregnancy. Current evidence suggests that use of Doppler reduces the risk of perinatal deaths and results in fewer obstetric interventions.
There is no evidence to support the following practices to reduce the risk of preterm birth in multiple pregnancy:
Bed rest
Prophylactic cervical cerclage
Ultrasound indicated cervical cerclage
Vaginal progesterone therapy
Intramuscular progesterone
Tocolytic therapy
Monozygotic twins have a 50% increase in structural abnormalities per baby and therefore a 20 weeks’ gestation anomaly scan is very important in the management of twin pregnancy. Serial growth scans are necessary to monitor fetal growth.
For uncomplicated monochorionic twin pregnancy the frequency of antenatal visits should be every 2–3 weeks from 16 weeks and an ultrasound examination should be performed to estimate fetal weight and look for TTTS. Antenatal visits for uncomplicated dichorionic twins should be every 4 weeks from 24 weeks until 36 weeks and weekly thereafter.
In the event of in utero demise of one twin in a monochorionic twin pregnancy, neurological injury may occur in the surviving twin.
Key Actions
Timing of Delivery
With close fetal surveillance, perinatal morbidity can be minimised by allowing uncomplicated monochorionic twin pregnancies to continue up to 37 weeks’ and dichorionic twins up to 38 weeks’ gestation.
In monochorionic twins the risk of late in utero death is 1.5%.
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