[Diagnosis of chorionicity].
BACKGROUND
The recognition of mono- and dichorionic twins is highly important and should be performed as early as possible during the pregnancy. This is justified by major differences between these two types of pregnancies for fetal mortality, morbidity, features of the complications, and appropriate follow up is required.


OBJECTIVE
To determine the modalities for the diagnosis of chorionicity.


METHODS
Articles were searched using PubMed, Embase and Cochrane library.


RESULTS
The diagnosis of chorionicity is appropriately made by the ultrasound examination. Before 10 weeks of gestation, the appearance and count of the gestational sacs is the best method (ELNP2). During first trimester ultrasound examination at around 12 weeks of gestation, the lambda sign is the most relevant sonographic sign (ELNP2). The diagnosis of chorionicity should be mentioned in the report of the first trimester scan. The diagnosis of chorionicity performed at first trimester is the reference for the rest of the pregnancy. Indeed, further determination of the chorionicity at second or third trimester is less accurate and more difficult. It should be performed by a combination of sonographic signs (fetal sex, twin peak sign, location of the placenta(s), or counting the layers of the amniotic membrane) (ELNP2). When the diagnosis of chorionicity cannot be obtained, the patient should be offered a second scan in a reference center (expert opinion).


CONCLUSIONS
The knowledge of the chorionicity is necessary for appropriate follow up of twin pregnancies. Ultrasound examination is a reliable method for the diagnosis of chorionicity early during the gestation.
Benoist
G
G
CHU de Caen, Service de gynécologie-obstétrique et médecine de la reproduction, avenue Georges-Clemenceau, 14000 Caen, France. benoist-gu@chu-caen.fr
Herlicoviez
M
M
D002823:Chorion; D004200:Diseases in Twins / Q000175:diagnosis*; D005260:Female; D005315:Fetal Diseases / Q000175:diagnosis*; D006801:Humans; D017410:Practice Guidelines as Topic; D011247:Pregnancy; D011248:Pregnancy Complications / Q000175:diagnosis*; D059285:Pregnancy, Twin; D011296:Prenatal Diagnosis