CHORIONICITY IN TRIPLET PREGNANCIES

Important reasons for determining the chorionicity of triplet pregnancies are:

1. Fetal reduction is usually carried out between 8-12 weeks. The presence of placental vascular anastomoses in monochorionic or dichorionic triplets may result in damage to the remaining fetuses.
2. Selective fetocide in the second trimester for a fetal anomaly requires the accurate determination of chorionicity.
3. The complications of monochorionic placentation are more severe in triplet than in twin pregnancies.

ULTRASOUND ASSESSMENT OF CHORIONICITY

1. "Twin peak" sign.
2. Thickness of the intertwin membrane.
3. Assessment of the membranes at the ipsilon zone.
4. Fetal sex not useful as at least 2 of the 3 will have the same sex

FIRST TRIMESTER

• Trichorionic:
  The gestational sacs are widely separated by thick choriodecidual tissue and have not yet approached each other to the Ipsilon Zone.

• Dichorionic Diamniotic – Embryo cleaved after day 8.
  Monochorionic pair are easy to identify in the same gestational sac.

• Dichorionic Triamniotic – Two embryos implanted; one cleaved prior to day 8.

SECOND TRIMESTER

• Trichorionic:
  All three dividing membranes are of similar thickness and echogenicity at the Ipsilon Zone.
• Dichorionic:
  One of the separating membranes is very thin (significantly thinner than the other membranes)
• Monochorionic:
  All three membranes are very thin and often difficult to identify.

Chorionicity and zygosity of 46 sets of spontaneously conceived triplets
Zygosity	Monozygotic	Dizygotic	Trizygotic
Placentation	%	%	%
Monochorionic Dichorionic Trichorionic	58% 33% 8%	- 71% 29%	- - 100%
Placentation	Monochorionic	Dichorionic	Trichorionic
Zygosity	%	%	%
Monozygotic Dizygotic Trizygotc	100% - -	19% 81% -	6% 39% 56%
Machin GA, Keith LG. An atlas of multiple order pregnancy. Parthenon publishing New York 1999;pp51.

REFERENCE

Above table adapted from:

Machin GA, Keith LG. An atlas of multiple order pregnancy. Parthenon publishing New York 1999..