Incidence and importance
There has been a sustained rise in the incidence of multiple pregnancy worldwide over the past few decades (1-3). This has largely been due to the rise in use of assisted reproductive techniques including in vitro fertilization (IVF), despite the adoption of a singleembryo transfer policy where appropriate in some countries (4).
One in eighty births following natural conception in the United Kingdom are multiples and one in four births after IVF (including intracytoplasmic sperm injection) in the United Kingdom result in either twins or triplets. The United Kingdom regulator estimates that about 40% of IVF babies are twins (5, 6). Triplet pregnancies continue to occur at a rate of 0.22 per 1000 livebirths, partly attributable to monozygotic twinning in assisted reproductive technique cycles and partly to spontaneous conception in women of advanced maternal age (7). The Human Fertilization and Embryology Authority (HFEA) reported that the number of women giving birth with multiple pregnancies (twins, triplets, and higher-order multiples) have more than trebled between 1992 and 2006 (from 664 to 2312) (2). This led to the introduction and subsequent implementation of the HFEA multiple pregnancy policy in 2008 with an aim to bring down the United Kingdom IVF multiple birth rate to 10% over a staged period (6, 8-10). The key recommendation to achieve such a reduction was to increase the proportions of elective single-embryo transfer (9). This has led to some reduction in multiple births, with one in four pregnancies being a multiple pregnancy in 2008 to one in six pregnancies in 2013 (9). However, monozygotic twinning may still occur with assisted reproductive techniques and elective single- embryo transfer, especially since the adoption of day 5 blastocyst transfer, to optimize successful pregnancy rates. Figure 20.1 shows a rise in multiple pregnancy over the years, with advancing maternal age.Multiple pregnancies are at increased risk for both mother (hypertensive disorders in pregnancy, diabetes in pregnancy, antepartum haemorrhage, obstetric cholestasis, anaemia, cardiovascular complications, postnatal illness, thromboembolic disorders, and maternal mortality) and the fetuses (miscarriage, preterm delivery, congenital malformations, stillbirth, and complications of monochorionic placentation such as twin-to-twin transfusion syndrome (TTTS)) (11). In general, maternal mortality associated with multiple births is 2.5 times that for singleton births. The overall stillbirth rate in multiple pregnancies is higher than in singleton pregnancies, ranging from 12.3 per 1000 twin births and 31.1 per 1000 triplet and higher-order multiple births, compared with 5 per 1000 singleton births. The risk of preterm birth is also considerably higher in multiple pregnancies than in singleton pregnancies, occurring in 50% of twin pregnancies. It has been shown that the rate of cerebral palsy is at least six times higher for twins and 18 times higher for triplets than for singleton babies (12).
Therefore, multiple pregnancies pose an increased workload to fetal medicine units (as well as obstetric services more generally and neonatal units). This includes early chorionicity assessment, recognition and counselling of the increased risks associated with monochorionic placentation, and fetal therapy where appropriate. Besides fetal medicine input, successful maternal and fetal outcomes can be achieved by provision of care for women with twin and triplet pregnancies by a nominated multidisciplinary team consisting of a core team of named specialist obstetricians, specialist midwives, and ultrasonographers, all of whom have experience and knowledge of managing twin and triplet pregnancies. Various national and international guidance on multiple pregnancy corroborate this approach (11). A 2015 Cochrane review, however, has concluded that the value of ‘specialized’ multiple pregnancy clinics in improving health outcomes for women and their infants has yet to be proven in appropriately powered, randomized controlled trials (13).