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There are three different types of twins:

Dichorionic diamniotic (DCDA) Twins

DCDA twins have two placentas and two separate amniotic sacs. These babies usually have different genetic material; approximately 10% will be identical.

Image: DCDA Twins


DCDA twins are usually routinely scanned monthly to assess growth and well being of each baby and for cervical assessment.

Monochorionic diamniotic (MCDA) Twins

MCDA twins have a single placenta and two separate amniotic sacs. These babies have identical genetic material.

Image: MCDA Twins


MCDA twins are routinely scanned every two weeks from 16-32 weeks to monitor for the early signs of twin-twin transfusion syndrome and can encounter other problems during pregnancy, including:

  • Acute twin-twin transfusion
  • Twin embolisation syndrome
  • Twin-reversed arterial perfusion syndrome or Acardiac twin

Monochorionic monoamniotic (MCMA) Twins

MCMA twins have a single placenta and both babies share the same amniotic sac. They also have the same genetic material.

Image: MCMA Twins


MCMA twins are scanned weekly after 24 weeks until elective delivery — usually at 32 weeks — in a major centre with a level 1 neonatal intensive care unit.  MCMA twins can be at a greater risk for:

  • Cord entanglement and loss of one or both fetuses
  • Acute twin-twin transfusion
  • Conjoined twins

Challenges facing all twins

All twins may potentially encounter the following special problems during pregnancy:

  • Premature delivery. The mean gestational age of delivery for twins is 36 weeks, for triplets is 33 weeks and for quadruplets is 29 weeks.  Half of all twin pregnancies will deliver prior to 36 weeks. Ultrasound can be used to monitor cervical length, which is much more accurate than pelvic clinical examination. Cervical length can identify those patients at high risk of premature delivery.
  • Growth restriction of one or both fetuses.
  • Low birth weight less than 2500 g. These infants are at significant risk of short-term and long-term health problems.
  • Greater risk for chromosomal and structural abnormalities.
  • Greater risk of fetal loss.

Your doctor will decide how often he or she wants your pregnancy monitored with ultrasound. If there is a problem identified in the fetus or with the growth, further investigations may be discussed with you, including prenatal testing such as amniocentesis or CVS.