Pregnancies with three or more fetuses are referred to as multifetal. The majority are polyzygotic (non-identical) because each embryo is derived from fertilisation of a different ovum. In monozygotic (identical) pregnancies a zygote, formed from the union of one ovum and one sperm, undergoes a division to form two or more genetically identical individuals.

In multifetal pregnancies the fetuses are usually non-identical but in some cases there may be co-existence of identical and a non-identical fetuses.

• How common are multifetal pregnancies?

Normally, twins occur in about one in 50, triplets in one in 6000 and quads in one in 500,000 pregnancies. In the last two decades however with the increasing availability of assisted reproductive technologies the rate of triplet and other multifetal pregnancies has risen dramatically. In about 10 per cent of pregnancies achieved by such techniques there are two or more fetuses.

• What are the risks with multifetal pregnancies?

Multifetal pregnancies are associated with a high risk of miscarriage (delivery before 24 weeks) and severe preterm delivery at 24-32 weeks. The majority of babies that are born free term now survive and develop normally. Sadly some of the babies that are very premature die and others become handicapped.

The chances of an adverse outcome depends on the gestation at delivery. Survival increases from less than 10 per cent before 24 weeks to more than 95 per cent by 32 weeks. The risk of severe handicap in those babies that survive decreases from about 50 per cent for those born at 24 weeks, to less than five per cent by 32 weeks.

In addition to the risks of miscarriage and preterm delivery, there is also a high risk for many pregnancy complications in multifetal pregnancies and their frequency increases with a number of fetuses.

• Embryo reduction

One of the options in the management of multifetal pregnancies is embryo reduction to twins. The aim is to increase the chances of survival and decrease the risk of handicap.

In pregnancies with four or more fetuses there is evidence that embryo reduction to twins is associated with a decrease in the risk of miscarriage and severe preterm delivery. In triplet pregnancies there is controversy concerning the possible benefits of reduction.

Embryo reduction involves the introduction of a thin needle into your abdomen and the injection of a chemical into the chest of one or more fetuses, which will result in their death. The dead fetus and placenta do not get removed from the uterus but they gradually, over a period of about three months, disintegrate and become part of the surviving placenta.

The greater the amount of dead fetal placental tissue the high at is the chance of complications and therefore it is best to carry out the embryo reduction at 11 to 13 weeks. This is the earliest gestation for ultrasound examination to determine if any of the fetuses have abnormalities or poor growth.

The greatest risk of miscarriage after a reduction is within one week of the procedure. However the risk persists for several months and is due to the breakdown of the dead fetal placental tissue.

Multifetal pregnancies are associated with a high risk of miscarriage at 24-32 weeks.

In the management of multifetal pregnancies there are three options:

• Continuing with the whole pregnancy

• Termination of the whole pregnancy

• Embryo reduction to twins

In pregnancies with four or more fetuses there is evidence that embryo reduction to twins is associated with a decrease in the risk of miscarriage and severe preterm delivery. In triplet pregnancies there is controversy concerning the possible benefits of reduction.

The chance of miscarriage is about one per cent in singleton pregnancies, two per cent in twins, three per cent in triplets and eight per cent in triplets reduced to twins.

The chance of preterm delivery at 24 to 32 weeks is about one per cent in singleton pregnancies, five per cent in twins, 20 per cent in triplets and 10 per cent in triplets reduced to twins.

In non-identical triplet pregnancies, irrespective of whether embryo reduction is carried out or not, there is a more than 90 per cent chance of healthy live births. However, reduction results in a slightly lower chance of severe handicap (about two per cent per baby in non-reduced triplets to less than one per cent per baby in those reduced to twins).