Twins !! Take a breath.
Extra roles of ultrasound in twin / multiple pregnancy.
After ultrasound’s first role in identifying a multiple pregnancy, it has a very important role in identifying the type of twins. This is easier and more accurate in the first trimester as the placenta which is also known as the chorion (which forms the outer sac) is easily distinguished from the amnion (a thinner inner-sac membrane). However in very early pregnancy the inner membrane is so thin it can be hard to see at all, and the number of yolk sacs is generally a better guide up to say 7 or 8 weeks.
Parents will be interested in whether their twins are fraternal (non-identical) which are the majority, or identical. Non-identical twins are the ones that vary with the environment – mother’s age, race, some medication such as ovulation induction.
Non-identical (fraternal) twins are the commonest. Since they started from 2 separate eggs they always have separate placentas even if the placentas are fused. As the inner sacs are formed within each outer chorionic sac, they automatically have two of these as well – so medically they belong to the dichorionic diamniotic group. These twins do not share blood vessels.
Identical twins can be dichorionic diamniotic just like non-identical twins, if the embryo splits into 2 very early. About 1 in 3 sets of identical twins develop like this. In that case, we do not get to distinguish them but medically they carry the same risks as the fraternal twins.
Most identical twins (two thirds) have one placenta and two amniotic sacs. Although they usually do well, they are a higher risk group, partly because of their shared blood circulation. This group receive much closer monitoring than the more common dichorionic twins.