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CHORIONICITY IN
TRIPLET PREGNANCIES |
Important reasons for determining the chorionicity of triplet pregnancies are:
- 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.
- Selective fetocide in the second trimester for a fetal anomaly requires the accurate determination of chorionicity.
- The complications of monochorionic placentation are more severe in triplet than in twin pregnancies.
ULTRASOUND ASSESSMENT OF CHORIONICITY |
- "Twin
peak" sign.
- Thickness
of the intertwin membrane.
- Assessment
of the membranes at the ipsilon zone.
- Fetal sex not useful as at least 2 of the 3 will have the same sex
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FIRST TRIMESTER |
- Trichorionic:
The gestational sacs are widely separated by thick choriodecidual tissue and have not yet approached each other to the Ipsilon Zone.
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Trichorionic Triplet Pregnancy |
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- Dichorionic
Diamniotic – Embryo cleaved after day
8.
Monochorionic pair are easy to identify in the same gestational sac.
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- Dichorionic Triamniotic – Two embryos implanted; one cleaved prior to day 8.
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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
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Zygosity |
Monozygotic |
Dizygotic |
Trizygotic |
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Placentation |
% |
% |
% |
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Monochorionic Dichorionic Trichorionic |
58% 33% 8% |
- 71% 29% |
- - 100% |
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Placentation |
Monochorionic |
Dichorionic |
Trichorionic |
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Zygosity |
% |
% |
% |
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Monozygotic Dizygotic Trizygotc |
100% - - |
19% 81% - |
6% 39% 56% |
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REFERENCE |
Above table adapted from: