Related Experiment Video
Updated: Aug 13, 2026

Guide Wire Assisted Catheterization and Colored Dye Injection for Vascular Mapping of Monochorionic Twin Placentas
Published on: September 5, 2011
Number of yolk sacs does not predict amnionicity in early first-trimester monochorionic multiple gestations
O Shen1, A Samueloff1, U Beller1
1Department of Obstetrics and Gynecology, Shaare Zedek Medical Center, Faculty of Health Sciences, Ben-Gurion University of the Negev, Jerusalem, Israel.
Objective:
To determine the relationship between amnionicity and number of yolk sacs before 11 weeks of gestation.
Methods:
Twenty-two cases of monochorionic multiple pregnancy were scanned before 11 weeks of gestation. There were 21 sets of twins and one of triplets. Amnionicity was determined by visualization of a dividing amniotic membrane between the gestational sacs. The number of yolk sacs was recorded and compared with the presence or absence of a dividing membrane for all fetuses.
Results:
In 17/20 (85%) cases of monochorionic diamniotic twins, two yolk sacs were seen. In 3/20 (15%) cases of monochorionic diamniotic twins, a single yolk sac was seen. In the one case of monochorionic diamniotic triplets, two yolk sacs were visualized. In one case of monoamniotic twins, a single yolk sac was observed.
Conclusions:
In monochorionic pregnancies, the presence of two yolk sacs predicts diamnionicity. However, the use of the number of yolk sacs as a predictor of amnionicity may not be accurate in a small proportion of patients. The diagnosis of monoamnionicity can be made only following a careful search for a dividing amniotic membrane.
Related Concept Videos
Cleavage and Blastulation
Nondisjunction
The Y Chromosome Determines Maleness
Evolution
Around 300 million years ago, the two sex chromosomes diverged from two identical autosomal chromosomes. Over time, the Y chromosome has lost most of its genes, shrinking in size. Today,...
Meiosis vs. Mitosis
Before the start of mitosis and meiosis I, the cell synthesizes DNA, resulting in two homologous copies of each chromosome. DNA synthesis is...
Nondisjunction
Zygotic Development And Stem Cell Formation

