A pathologist is examining five placentas from five different bihs. He notes the following characteristics in the five placentas: Patient A: fused dichorionic diamniotic Patient B: dichorionic diamniotic Patient C: circumvallate placenta Patient D: monochorionic diamniotic Which of the patients unquestionably gave bih to identical twins?

Patient D is the lucky mother of identical (monozygotic) twins. The chorion forms before the amnion, so the possible combinations are monoamniotic and monochorionic; diamniotic and monochorionic; and diamniotic and dichorionic (either fused or separated). The first two of these possibilities are seen only in identical twins. However, the latter can be seen either in fraternal or identical twins. Exactly what happens in the case of identical twins depends on the precise point at which twinning occurs: very early separation produces completely separate membranes with duplication of both chorion and amnion; somewhat later separation produces one chorion and two amnions; and very late separation produces one chorion and one amnion. A dichorionic, diamniotic placenta develops if splitting occurs early after feilization, before the chorion forms. This type of placenta may occur with either monozygotic or dizygotic twins. Thus, we are unable to determine whether Patient A or Patient B had identical twins from the examination of the placentas. Patient C did not have a twin pregnancy. A circumvallate placenta has an extrachorial poion outside the inseion of the amniotic membranes, creating a circumferential groove (vallum in Latin). A bipaite placenta is composed of two equal segments. Circumvallate and bipaite placentas are examples of placental abnormalities that usually have no clinical significance. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 39. Multifetal Gestation. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.