Twins and more - Multiple Pregnancy

Types of multiple pregnancy and their importance (cont.)
For instance if the embryo divides into two in the first couple of days, each fetus will have a separate sac and placenta. If they split after more than five days they will share the sac and placenta (mono-chorionic). If they leave it too late to split they can end up as conjoined or siamese twins.

The reason this is important is because monochorionic twins account for roughly a third of twin pregnancies, but about two thirds of the increased risk associated with being a multiple pregnancy. For this reason monochorionic pregnancies tend to be monitored more closely in pregnancy. For instance, the risk of very preterm delivery (see Preterm labour and delivery), i.e. delivery before 32 weeks gestation is about 1% for singleton, 2% for dichorionic and 10% for monochorionic pregnancies. As preterm delivery is the biggest cause of perinatal death and handicap you can see the importance of knowing which type of twin pregnancy you have. An ultrasound scan at the end of the first trimester (11 - 13 weeks) is the best time to check on the chorionicity of a multiple pregnancy (see Ultrasound).