There is no doubt about it, twins are definitely something special. To many people the idea of twins is very attractive - not only are they very cute, but they also have the advantage of double the reward for only one pregnancy. Unfortunately it's not all gain/no pain - twin pregnancies are more risky for both mum and babies. Most complications of pregnancy are more common with twins (with the exception of going over the due date!) and probably the most common of these is being born early. The average birth gestation for twins is 36 weeks - approximately 4 weeks earlier than singleton pregnancies. The real concern however is those who are born before 32 weeks, because these babies are ones that have the most problems.
Kypros Nicholaides is a Professor of fetal medicine at King's College in London. When I was at medical school there, he was just building the department; now he has an empire, and is one of the major players in fetal medicine, having published a lot of the research into ultrasound screening for Down's Syndrome. Yesterday, I came across a new website of his with some nice images of twin pregnancies, along with a good description of the different types of twins. Twins can either be identical (monozygotic) or non-identical arising from two fertilised eggs (dizygotic) and two thirds of twins are non-identical.
The thing that obstetricians are most interested in when it comes to twins is how the placenta is arranged. Essentially, each baby in his or her own completely separate, dual-layer sac is best (called dichorionic), whereas sharing a single one (monoamniotic) is least favourable. Fortunately, monoamniotic twins are quite rare and indeed most twin pregnancies have a perfectly healthy outcome.
Kypros has other websites about his Fetal Medicine Centre and another site for heath professionals about Down's screening, including a free download of his 11-13 week scan book.
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