Multiple pregnancies have their own problems at each stage of a pregnancy. Twins are the most common multiple pregnancy, with triplets and quadruplets etc. much less common. One of the reasons for an increase in the attention afforded multiple pregnancy has been the rise in these pregnancies as a result of assisted conception and ovulation stimulation techniques.
Types of multiple pregnancy and their importance
Twins and more can be identical or mono -zygotic (where the twins come from the same egg and sperm) or non-identical or dizygotic (where they arise from two or more eggs and sperm). When non-identical twins implant in the womb they always have two separate sacs and placentas (dichorionic, chorion being one of the sac membranes). If the twins are identical, they may or may not share a sac; it depends how long after conception the split takes place.
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.
Multiple pregnancies are at greater risk of:
- Chromosomal abnormalities (see Prenatal diagnosis). Dizygotic twins are more common in older women, who have a greater risk of chromosomal problems, and each egg carries its own risk. Special counselling is advisable regarding this issue in multiple pregnancy.
- Fetal abnormalities. The risk for the non-identical (dizygotic) twins is the same as a singleton, but identical twins have a four-fold increase in the risk of finding an abnormality.
- Common obstetric problems. There is an increased risk of developing antepartum and postpartum haemorrhage (see Haemorrhage), hypertension www.pregnancycare.co.uk/hypertension diabetes, anaemia and deep venous thromboembolism (see Major symptoms) among other problems in twin pregnancy.
- Death of one twin can affect the chances of the remaining twin staying alive or being handicapped.
- Twin-twin transfusion syndrome. This can happen if the twins share the same sac and placenta. One fetus gets too much blood, becomes big and has extra fluid around it (polyhydramnios), while the other receives too little, is small and has no fluid around it (oligohydramnios – stuck twin). If the extra fluid around the recipient (big twin) becomes too much for the uterus the woman presents with a rapidly swelling, painful abdomen. She may require drainage of the fluid or laser to the vessels communicating from one side of the placenta to the other.
Because many twins deliver early, and because it is not considered safe to allow a multiple pregnancy to labour if there has been previous uterine surgery, e.g. Caesarean section, the section rate for twins is higher than for singletons. If the delivery is vaginal, the experience of the operator is essential in ensuring that not only the first, but also the second twin is delivered safely.