Different kinds of twins

Why is it important to distinguish monochorionic from dichorionic twins?

What are the risks of a twin pregnancy?

• A too early delivery

  This is the most common problem of a twin pregnancy. About half of twins are born before 37 weeks. When born after 32 weeks, babies may still have feeding and breathing problems, but they will nearly always survive. In contrast, babies born between 24 and 32 weeks have a high risk of dying or handicap if they survive. The median gestational age of delivery is only marginally earlier in monochorionic (36 weeks) compared with dichorionic twins (37 weeks). However, the proportion of pregnancies delivering very early (before 32 weeks) is nearly twice as high in monochorionic (9%) compared with dichororionic twins (5.5%).
  
  

• Poor growth of the babies

  In twins, the risk of delivering growth-restricted babies is about 10 times higher than in singletons. The chance of both babies growing poorly is about 4 times as high in monochorionic (7.5%) compared with dichorionic (2%) pregnancies. It may also happen that that one baby is much smaller than the other. This problem seems to affect monochorionic twins (11%) as often as dichorionic (12%) twins, although the underlying cause, the consequences and the management are different. As mentioned above, most dichorionic twins are fraternal and thus genetically different. As such, genetic causes may explain the growth difference. Also, both twins have their own placenta and it may be that one placenta does not work as well as the other. In contrast, monochorionic twins are carrying the same genes and are on the same placenta, so there must be other reasons why one baby grows better than the other. Firstly, the placenta is often not equally divided between both twins with usually the bigger twin having the largest part of the placenta. Secondly, because the blood circulations of both babies are connected in the single placenta, one baby may continuously transfer small amounts of blood and thus oxygen and nutrients to the other and therefore grow poorly. If growth is faltering for one or both babies, then their well being will be followed closely.
  

• Twin-twin transfusion syndrome (TTTS): a complication unique to monochorionic twins:

  In monochorionic twins, the blood vessels that connect the blood circulations of both babies will cause no problems, as long as the distribution of blood supply is well balanced. However, may happen that more blood is directed from one baby towards the other, leading to TTTS. The first baby is called then the donor since he essentially pumps his blood into the second baby, who is called the recipient. The donor usually suffers from anaemia (too little blood), hypovolemia (too little fluid) and is frequently smaller. Due to a deficiency in blood and fluid, the donor will produce less urine. Since urine is the main constituent of the amniotic fluid, the amniotic fluid around the donor will decrease (oligohydramnios), eventually disappearing altogether. The donor will appear to be almost shrink-wrapped and ‘stuck’ up against the wall of the womb, which explains why the donor is often called the ‘stuck twin’. Meanwhile, the recipient has his own worries. He receives too much blood and eliminates the excess fluid by increasing his urine production. This causes too much amniotic fluid around the recipient (polyhydramnios) and a rapid and excessive distension of the womb, leading to a too early delivery or rupture of the membranes. TTTS complicates about 15% of monochorionic twin pregnancies usually between 16 and 24 weeks of gestation. If TTTS is not picked up and left untreated, it is likely to cause loss of both babies. The two currently most performed treatments are drainage of the excessive fluid or surgical separation of the connecting vessels on the placenta is performed by means of laser.
  

Summarising