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Multiple pregnancies now account for 1.5% of all pregnancies. 

The commonest form of multiple pregnancies are twins, though triplets, quadruplets and order higher order multiple pregnancies are occasionally seen as well. Such higher order multiple pregnancies have even higher risks than twins. 

In twins, there are 3 main types of placentation: 

  • Dichorionic diamniotic (DCDA) - "Di" stands for 2, "chorion" refers to the placenta, and "amnion" refers to the amniotic sacs. Hence DCDA twins refer to twin pregnancies with separate placentas and separate amniotic sacs. The majority of DCDA twins are fraternal (i.e. non-identical twins) though a small percentage of the liked-sex DCDA twins are identical. 

  • Monochorionic diamniotic (MCDA) - "Mono" stands for 1. Hence MCDA twins refer to twin pregnancies with a single shared placenta and separate amniotic sacs. Almost all MCDA twins (with very few exceptions) are identical. 

  • Monochorionic monoamniotic (MCMA) - MCMA twins refer to twin pregnancies with a single shared placenta and single amniotic sac. These are identical twins as well. Such twin pregnancies are associated with the highest risk as there is a high probability of cord entanglement between the twins caused movement and entanglement of the cords between the 2. 

The diagnosis of the chorionicity is easily achieved by ultrasound scan between 7-14 weeks as the presence of a twin peak sign denotes DCDA twins while the absence of a twin peak sign denotes MC twins. 

Due to the shared vascular anastomoses in a MC placenta, MC twins have additional specific vascular risks of: 

  • 10-15% risk of twin twin transfusion syndrome (TTTS) 

  • 10-15% risk of selective intrauterine growth restriction (IUGR) 

  • 2-5% risk of twin anaemia polycythaemia sequence (TAPS) 

  • Risks of death / brain damage to co-twin if one twin dies

In addition, both DCDA and MC twin pregnancies are also at increased risks (compared to singleton pregnancies) of: 

  • exaggerated symptoms and signs of pregnancy e.g. nausea and vomiting

  • fetal anomalies 

  • preterm labour (PTL) 

  • intrauterine growth restriction (IUGR) 

  • stillbirths (SB) 

  • pre-eclampsia 

  • gestational diabetes mellitus

Twin pregnancies, especially monochrionic twins, are high risk pregnancies that have to be monitored closely with ultrasound. It is recommended that monochorionic twins and dichorionic twins are monitored every 2 weeks from 16 weeks and 2-3 weeks from 20 weeks respectively.

Progesterone pessaries may be useful to reduce spontaneous preterm delivery amongst twins with short cervix.

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