Multiple pregnancies and preterm birth

Multiple pregnancies make up about 1.5 per cent of births in Australia, with about 4300 sets of twins and just over 80 sets of triplets and higher order multiples born each year.

Multiple pregnancies are usually recognised by the end of the first trimester due to the widespread use of ultrasound. Earlier diagnosis of twin pregnancy is helpful in determining level of risk, based primarily from the type of placenta.

Two placentas (dichorionic): this is the most common form of placentation in twin pregnancies and is associated with a lower complication risk.
Single placenta (monochorionic): the twins share a single placenta and are at increased risk of congenital malformations, growth disorders and twin-twin transfusion syndrome. Usually each twin has its own sac (diamniotic) but rarely the twins share a single sac (monoamniotic) which places the pregnancy at risk from complications of umbilical cord entanglement.

Overall, twin pregnancies are much more likely to be complicated by preterm birth (>50%) than single ones and for the babies to be of low birth weight. This risk is increased even more in higher order multiple pregnancies, such that virtually all triplet pregnancies are born preterm (average 32-34 weeks gestation).

Being born preterm places the twins at increased risk of lung problems, infection, brain development problems and death.

Women with multiple pregnancies require an increased level of antenatal care to optimise outcomes. For monochorionic twin pregnancies the level of ultrasound surveillance is very high, typically every two-weeks from mid-pregnancy onwards to allow for timely recognition of complications should they occur. Uncomplicated dichorionic twin pregnancies usually require ultrasound assessment every 4-weeks.

Preterm birth is a common complication of twin pregnancies and unfortunately the current prevention treatment strategies used in single pregnancies do not appear to be as effective in multiple pregnancies. This is probably because the mechanism of preterm birth in multiple pregnancies is often different to that in singletons. Further research is needed to investigate the mechanisms of preterm birth in multiple pregnancies before new medical treatments can be developed.

Twins are also at increased risk of other pregnancy complications that may require a preterm delivery, such as poor growth of one or both babies.

It is important that an individualised antenatal care plan is developed for women with a multiple pregnancy and that this care is provided by health care providers and medical units with the facilities available to manage problems should they arise.

Contact details

Women and Infants Research Foundation
Carson House, King Edward Memorial Hospital
374 Bagot Road, Subiaco, WA 6008

Telephone: 08 6458 1437
Fax: 08 6458 1642
Email: info@wirf.com.au

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Multiple pregnancies and preterm birth

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