Diamniotic, Mono and Dichorionic Twins
|Monochorionic, Diamniotic Twins|
Dichorionic (two placenta) monozygotic twinning occurs approximately 20-25% of the time with monchorionic (single placenta) twins occuring 75-80% of the time in these pregnancies. As was alluded to above, the distinction between monochorionic and dichrorionic twins is important and may be difficult. Early in pregnancy this distinction may be quite easy as the gestational sacs in dichorionic twins may be widely separated by a thick band of interposed chorionic tissue (the developing placenta), as in the case above. Later in pregnancy, the differentiation between mono and dichorionic twins is dependent upon observations of fetal sex, intertwin membrane thickness and number and location of placental masses. Twins with different placental sites and of different gender will have dichorionic placentation. When the fetuses are of the same sex or when a single placental site is seen the twins may either be mono or dichorionic. In these instances, the determiniation of chorionicity will depend upon the interpretation of membrane thickness.
|Dichorionic twins (chorion-large arrow) Amnion- small arrow)|
In the case of monochorionic twins, the "dividing membrane" seen at the time of ultrasound is the result of two layers of adjacent amnion from each gestational sac. This membrane is relatively thin. When the pregnancy is dichorionic the "dividing membrane" will be thick, comprised of two layers of amnion and two interposed layers of chorion. Early in pregnancy this differentiation is quite easy, however this differentitation may be quite difficult later in pregnancy. Accuracies > 85% are often achieved for dichorionic twins even late in pregnancy. The thin membrane in monochorionic twins may be more difficult to identify at all. Various groups have utilized methods such as the subjective appearance of membrane thickness, actually attempting to measure the number of layers and objectively measuring the "dividing membrane" (with a cutoff of 2 mm). Even with the best of techniques, inter and intraobserver error may be quite high.
|Dichorionic twins (twin-peak sign)||Triplets / Trichorionic with "twin-peak" sign (large arrow) and relatively thin "dividing membrane" (small arrow)|
In 1992, Finberg made the observation that in dichorionic twins with a single placental "mass", a triangular projection of placental tissue beyond the placental surfaces could be seen extending between the layers of the intertwin membrane.He coined the term the "twin-peak" sign. This has proven to be quite useful, when seen, in cases of dichorionic gestation especially when the intertwin membrane is relatively thin. Thus even if a discrete "twin-peak" cannot be seen, intertwin membrane thickness should probably be assessed near the placental edge, rather than in mid-uterine cavity.
Stagiannis KD, Sepulveda W, Southwell D, Price DA, Fisk NM. Ultrasonographic measurement ofthe dividing membrane in twin pregnancy during the second and third trimesters: A reproducibility study. Am J Obstet Gynecol 173:1546-1550, 1995
Finberg HJ. The "twin peak" sign: Reliable evidence of dichorionic twinning. J Ultrasound Med 11:571-577, 1992
Mahony BS, Filly RA, Callen PW. Amnionicity and chorionicity in twin pregnancies: Prediction using ultrasound. Radiology 155:205-209, 1985
Winn HN, Gabrielli S, Reece EA, Roberts JA, Salafia C and Hobbns JC. Ultraonographic criteria for the prenatal diagnosis of placental chorionicity in twin gestations. Am J Obstet Gynecol 161:1540-1542, 1989
Wood SL, St. Onge R, Connors G., Elliot PD. Evaluation of the twin peak or lambda sign in determining chorionicity in multiple pregnancy. Obstet Gynecol 88:6-9, 1996
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