The chorionicity of a twin pregnancy can be assessed by the 5th week of gestation. The number of embryos can be determined by the 6th week of gestation with the onset of cardiac activity. Determination of amnionicity has to wait until the 8th week of gestation when enough amniotic fluid is present in the amniotic sac to separate the amnion from the embryonic body. Determination of chorionicity and amnionicity during the second and third trimester may be accomplished by: a; counting the number of placental sites b; imaging the gender of the fetuses c; evaluating the intertwin membranes for the presence of the 'lambda' or 'twin-peak' sign or 'T-shape' take-off, the thickness of the intertwin membrane and the number of layers present. In the ideal world all patients should have a first trimester transvaginal scan since TVS can provide an accurate and quick diagnosis of the chorionicity and amnionicity of a twin pregnancy. During the second and third trimesters determining the chorionicity and amnionicity may be possible, but it will take time and effort and not all cases will be correctly classified.