Solution: Twins occur spontaneously in approximately 1 out of every 80 livebirths. Determining amnionicity and chorionicity is important and can usually be done if you know what to look for. Lets review. Dichorionic-diamniotic: In this setting, there will be two distinct gestational sacs. Each will have an embryo and a yolk sac and there will be a thick membrane separating the two. The "lambda" or chorionic peak sign as seen in this case is virtually diagnostic of dichorionicity. These can be identical or fraternal twins. Monochorionic-diamniotic: In this case, there will appear to be one single gestational sac with two embryos because there is only one outer sac, the chorion, but each twin is actually in its own inner sac, amnion and therefore will each have its own yolk sac. High resolution transducers, or transvaginal imaging may be necessary to see the separating membrane (crucial since amniocity needs to be confirmed. These are almost always identical twins. Monochorionic-Monoamniotic: This is a high risk pregnancy with approximately 50% mortality. There will be two embryos in a single gestational sac and there will only be on yolk sac and no intervening membrane. This situation can also lead to conjoined twinning and it is therefore important to look for this possibility. Intertwined limbs, or amniotic cords can be seen in this setting and sometimes one can see the different heart rates within intertwined cords using doppler. In addition to twinning, this patient has a septate or bicornuate uterus (See figure 3 with twin on one side and normal endometrium in the other side of the uterus (transverse view)). This was not fully worked up as of yet, but the patient did note that they had a difficult time getting pregnant.