Chorionicity ultrasound

A chorionicity ultrasound is used to distinguish between twins who 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

Reference:

1. NICE (April 2024). Twin and triplet pregnancy.