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Multiple pregnancy – Non-identical and Identical twins

Multiple pregnancies can be exciting but they are associated with more problems and require close supervision.

With multiple pregnancies we are usually referring to Twins, and the discussion will focus on twin gestations only. Higher order multiples such as triplets are far less common.

Twins are usually Non Identical (approx. 1-2 %) or Identical (approx. 0.1%).

Twins are more common now given the increasing frequency of assisted fertility treatments for couples with fertility problems and also advancing maternal age. These are usually non identical twins.

Non Identical twins (Dizygotic)

Dizygotic twins result from the fertilization of two separate eggs by two separate sperm. Both pregnancies are different genetically and are both independent with their own separate placentas (Diamniotic Dichorionic).

Dizygotic twins are higher risk pregnancies with increased incidence of:

  • Intrauterine growth restriction
  • Premature labour
  • Premature rupture of membranes
  • Antepartum Haemorrhage
  • Hypertension/Pre eclampsia
  • Gestational diabetes
  • Malpresentations

In view of these potential problems 60% of non identical twins will be delivered before 36 weeks gestation. The mode of delivery will be determined by the gestational age, the potential risks and the presentation of the first twin.

In normal twin gestations it is common to have a normal vaginal delivery.

In complicated twin pregnancies it is likely that the delivery will be by caesarean section depending on the nature of the risk.

Identical twins (Monozygotic)

Monozygotic twins occur when one fertilized egg splits into two separate zygotes. Each one is genetically the same. In decreasing order of frequency they can be divided into three groups:

  • Diamniotic Dichorionic (70–75 %)
  • Diamniotic Monochorionic (25-30%)
  • Monoamniotic Monochorionic (1%)
  • Rarely Conjoined twins (1:60,000)

Identical twins are very high-risk pregnancies. The risks for non identical twins are high but they are also compounded by additional risks especially in Monochorionic pregnancies. These pregnancies carry a 15% risk of twin to twin transfusion syndrome (TTTS) and the less common complications such as TRAP sequence.

Identical twins require ultrasound scanning every two weeks from 16 weeks to identify TTTS. Identical twins will require earlier delivery and that is usually by caesarean section.