Chorionicity is the number of placentae of a pregnancy. This is most accurately determined between weeks 6 and 9 of gestation, sometimes up to week 12. At this time, a thick membrane (two chorions and two amnions) is clearly visible at ultrasound, separating dichorionic twins, which joins the placenta to form a lambda shape. In monochorionic twins, this membrane (two amnions only) is much thinner and joins the placenta to form a “T” shape. Ultrasounds taken later in gestations are less reliable due to the increased crowding of twins in the uterus.

Physical examination of the inter-twin membranes at birth should also be used to determine chorionicity. This will provide confirmation of early ultrasound data and to determination of chorionicity in twins without early ultrasounds information. Dichorionic membranes are thick, opaque and can be pulled apart, whereas monochorionic membranes are thin and semi-transparent.

TRA Types Of Twinning Image

© A/Prof Mark Umstad and A/Prof Jeff Craig

For a review of the mechanisms and evidence for typical and atypical twinning, please click here

Twins Research Australia

Address: 3/207 Bouverie St
Carlton, Vic 3010

Freecall: 1800 037 021

Email: info@twins.org.au

ABN: 84 002 705 224

Twins Research Australia is a national resource supported by a Centre of Research Excellence Grant from the National Health and Medical Research Council and housed within the University of Melbourne.

Privacy Policy

 

Follow or ‘like’ us