The twin challenge in prenatal diagnosis

Twin pregnancies have fascinated humans for centuries and have been at the centre of numerous legends and stories—from Romulus and Remus, the mythical founders of antique Rome to Eng and Chang, the original nineteenth century Siamese twins. Since ancient times, a magic conception has been associated with twins, either in a positive or a negative sense, but often with some kind of a ‘fatidic’ aspect. Twin births were considered to be events of extraordinary importance, but were also a source of fear. In medieval Europe, many people believed that twin pregnancies resulted from intercourse during pregnancy, and the second born twin was considered evil and was usually killed. There is evidence of medical interest in twin pregnancies throughout history. In his treatise ‘De superfoetatione’, Hippocrates describes monochorionic twins in a scientific manner with concepts similar to those of modern embryologists: ‘a woman, pregnant with twins, gives birth to them both at the same time, just as she has conceived them; the two infants are in a single chorion’. By contrast, Aristotle regarded twins as anomalies associated with notions of monstrosity and excess, an idea that prevailed among the medical establishment until the Renaissance. The first scientific classification of twins was published in the 1860s by Spaeth (1861–1862) who observed that around 25% of twins were born with a single placenta (monochorial) and the twins in these cases were of the same sex. The first description of twin-to-twin transfusion syndrome (TTTS) was by a German obstetrician, Friedrich Schatz, in 1875. Over the following 35 years, Schatz injected the fetal placental blood vessels of twins and showed clearly that there are inter-twin blood vessel anastomoses, most commonly artery-to-artery communications (Schatz, 1910). He also observed artery-to-vein communications. As these travelled from one twin to the other, the possibility of ‘transfusion’ from one twin to the other was suggested. These transfusions took place through a ‘villous district’ of the placenta and, because they could not be seen directly from a surface inspection of the blood vessels in the placenta, he called this the third circulation. He later described the classical TTTS in a very long and meticulous paper when he observed twins that were grossly discordant, one being oedematous, the other shrivelled up. The oedematous twin passed away in 12 h, urinated constantly and had a hugely distended bladder at autopsy. The other, growth-restricted, baby passed away in 53 h, never urinated and had an empty bladder. Schatz inferred that the polyhydramnios was due to excessive intrauterine urination of the ‘recipient’ of the transfusion from the ‘donor’ twin who was atrophied and who he described as the ‘stuck twin’, because there was no fluid in which he could move before birth. It is now well recognised that mothers and fetuses in twin pregnancy are at higher risk of complications and adverse outcome than in singleton pregnancy. Twin pregnancies represent 12–15% of all premature deliveries and 15% of all neonatal death (Ananth et al., 2004; Glinianaia et al., 2002). Compared to singletons, twins have a four-times higher risk of developing cerebral palsy (Topp et al., 2004), and their mothers are at higher risk of death, mainly due to severe preeclampsia and placenta abruptio (Sibai et al., 2000). The management of twins is a constant challenge for the perinatal team, but there is clear evidence that specialised care improves the outcome of twins at birth and beyond (Luke et al., 2003; Vintzileos et al., 2003). This special issue of Prenatal Diagnosis includes a comprehensive series of reviews and a number of research articles on the diagnosis, methods of screening, complications and management of twin pregnancies. The methods of diagnosis are reviewed (Shetty and Smith) and the difficulties encountered in screening highlighted (Wald and Rish), with particular emphasis on the further complications that occur in pregnancies conceived by assisted reproductive technologies (ART) (Maymon et al.). Having identified a twin pregnancy at increased risk of Down syndrome, there are particular issues to consider when performing an invasive test (Weisz and Rodeck) or indeed managing the pregnancy when one twin is small (Kingdom et al.) or abnormal (Rustico et al.). There are also detailed reviews describing the rarer complications of multiple pregnancies (Evans et al.). The one describing the pathophysiology of TTTS brings the reader up-to-date with current thinking on the aetiology of this phenomenon (Galea et al.). Over the last few years there has been considerable debate as to how best to manage TTTS; that debate and its resolution are summarised clearly (Robyr et al.). Even less common is the complication of an acardiac twin, and Wong and Sepulveda describe well the sonographic diagnosis and management, and proposes a new classification. As suggested above, conjoined twins have fascinated man over the centuries. Wee and Jauniaux have reviewed the prenatal diagnosis and management of twin pregnancies complicated by hydatidiform mole. Advances in surgical techniques (Spitz) and sonographic diagnosis (Pajkrt and Jauniaux) have combined to improve the management of this condition. Finally, as we move into the twenty-first century there has been an increasing recognition of the psychological complications of pregnancy. Those affecting multiple pregnancies can be particularly complicated