Introduction

In the United States in 2004, 3.4% of all births were multiple births. Between 1994 and 2004, the multiple birth ratio in the United States increased by 32% (from 2.6% to 3.4%), largely due to increased use of assisted reproductive technology. This increase in multiple birth rates has had a tremendous impact on prematurity. In 2004, one in eight babies (12.5% of live births) was born prematurely; and of multiple gestations, 61.4% were born preterm, 58.5% were low birthweight (less than 2500 g), and 11.5% were very low birthweight (less than 1500 g). Infants from multiple gestations thus carry a tremendous economic burden, in 2005 the annual cost of prematurity in the United States was well over $26 billion.

Among multiple gestations, the highest incidence is contributed by twins. Besides prematurity, twins, regardless of type, are at higher risk for neurological injury. Over the years, several studies have correlated cerebral palsy with multiple gestations. A recently published large case–control study based on the Swedish Medical Birth Registry between 1984 and 1998 showed a 1.4 odds ratio (95% CI 1.1–1.6) of cerebral palsy in twin gestations relative to their singleton counterparts.

Besides their effect on society, twins pose interesting and challenging diagnostic and management dilemmas. From conception to delivery, and even beyond, twin gestations behave remarkably differently from their singleton counterparts, and have a myriad of unique physiologic changes and pathologic conditions that one must consider when caring for these special pregnancies.