Sunday, June 14, 2009

Dichorionic-Diamniotic
can be
Fraternal or Identical

The twin peak sign indicates the presence of a dichorionic-diamniotic twin gestation. The twin peak represents the extension of placental villi into the potential space that is formed from the reflection of apposed amniotic and chorionic layers from each fetus. It forms where two separate placentas grow contiguously and appear fused. The twin peak can be of variable size, and only its presence is required to suggest that the pregnancy is dichorionic-diamniotic. This sign is most useful in assessing the chorionicity of pregnancies after 10 weeks. Prior to this time, gestational sacs are readily recognizable and allow a rapid and accurate determination of chorionicity.


F2A.gif

The above drawing illustrates how the twin peak sign is reliable evidence of dichorionicity. In a dichorionic pregnancy, both the amnions and the chorions reflect away from the placental surface, creating a potential space into which villi can grow. Large arrow shows the resulting twin peak sign. Monochorionic diamniotic pregnancies have a single layer of continuous chorion (large arrow) limiting villous growth; the apposed amnions form a thin membrane separating the two amniotic cavities. A = amnion, C = chorion.



Monochorionic/Diamniotic
are
Identical


F2B.gif


The above drawing illustrates how the twin peak sign is reliable evidence of dichorionicity. In a dichorionic pregnancy, both the amnions and the chorions reflect away from the placental surface, creating a potential space into which villi can grow. Large arrow shows the resulting twin peak sign. Monochorionic diamniotic pregnancies have a single layer of continuous chorion (large arrow) limiting villous growth; the apposed amnions form a thin membrane separating the two amniotic cavities. A = amnion, C = chorion.

Courtesy of The Department of Radiology, Beth Israel Deaconess Medical Center, Boston, MA.



Monochorionic/Diamniotic Twins (mono/di)
Identical twin pregnancy ultrasound Ultrasound of another pregnancy at same gestational age as above, but with identical (monozygotic) twins one gestational sac is seen with 2 yolk sacs visible inside it.
This results from an early embryo dividing into 2 genetically identical embryos.


















5 Weeks Gestation



Monochorionic/Diamniotic Twins (mono/di)
The intertwin membrane is barely visible, look closely and you will see it just below the baby on the left.









10 Weeks Gestation

Monochorionic/Diamniotic Twins (mono/di)
Notice the membrane, it is only partially visible in this mono/di twin sonogram.
Photobucket
12 Weeks Gestation

Dichorionic/Diamniotic Twins
These ultrasound images demonstrate twin pregnancy at 13 weeks gestation. The intertwin membrane reveals that this is a diamniotic twin pregnancy. Also note the presence of the triangular wedge of placental tissue projecting into the intertwin membrane. This is called the Twin peak sign or the lambda sign which clearly suggests a Dichorionic twin pregnancy. Ultrasound images of Dichorionic gestation courtesy of Dr. Mohammed Sheriff, Tanzania.


























13 Weeks Gestation



Dichorionic/Diamniotic Twins
Transverse image of the placenta of a twin pregnancy at 17 weeks gestational age reveals a triangular peak of villi (short arrow) extending from the placenta into the intertwin membrane (long arrow): the twin peak sign. Amniocentesis confirmed the pregnancy to be dizygotic, with male and female fetuses.














17 Weeks Gestation


Additional Information

Journal of Ultrasound in Medicine, Vol 14, Issue 6 415-419, Copyright © 1995 by American Institute of Ultrasound in Medicine


JOURNAL ARTICLE

Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins

B. Bromley and B. Benacerraf
Department of Obstetrics and Gynecology, Massachusetts General Hospital, Harvard Medical School, Boston, USA.

The purpose of this study was to evaluate the relationship between the number of yolk sacs and amnionicity in monochorionic twin pregnancies scanned early in the first trimester. We retrospectively reviewed images of all monochorionic twins scanned between 6 and 9.5 weeks' gestation and with pathologic or sonographic confirmation of chorionicity-amnionicity. Each film was reviewed for the number of yolk sacs present, as well as for the gestational age at which the amniotic membrane was first visualized. Twenty monochorionic-diamniotic pregnancies and two monochorionic-monoamniotic pregnancies met the criteria for inclusion in the study. In diamniotic pregnancies scanned at less than 8 weeks' gestation, only the yolk sacs were identified; none of the dividing amniotic membranes were detected. Two yolk sacs were identified in all but one case. In this case, although one yolk sac was seen at 6 weeks, follow-up scanning at 8 weeks revealed twoyolk sacs. In each of the monochorionic-monoamniotic twin pregnancies, one yolk sac was seen at 9 weeks and a single amnion encircled both embryos. Weconclude that the sonographic identification of two yolk scas in monochorionic twins enables us to make the diagnosis of diamniotic twins early in the first trimester, before the amniotic membrane can be imaged. The presence of one yolk sac should prompt a follow-up ultrasonogram to assign amnionicity definitively.