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Chorionicity ultrasound

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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A chorionicity ultrasound is used to distinguish between twins that share a placenta and those who have separate ones, so that twins at higher risk of complications can be monitored more closely.

  • determine chorionicity and amnionicity at the time of detecting a twin or triplet pregnancy by ultrasound using:
    • the number of placental masses
    • the presence of amniotic membrane(s) and membrane thickness
    • the lambda or T-sign
  • if a woman with a twin or triplet pregnancy presents after 14+0 weeks, determine chorionicity and amnionicity at the earliest opportunity by ultrasound using all of the following:
    • the number of placental masses
    • the presence of amniotic membrane(s) and membrane thickness
    • the lambda or T-sign
    • discordant fetal sex

Chorionicity:

  • number of chorionic (outer) membranes that surround babies in a multiple pregnancy
    • If there is only 1 membrane, the pregnancy is described as monochorionic;
    • if there are 2, the pregnancy is dichorionic; and
    • if there are 3, the pregnancy is trichorionic
  • monochorionic twin pregnancies and monochorionic or dichorionic triplet pregnancies carry higher risks because babies share a placenta

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