The second twin: retained and unretained.

In this comparative review of retained and unretained second twins, the commonest causes of retention of the second twins, were found to be uterine inertia (48.4%) and obstruction from malpresentation and malposition (32.3%). The pair-presentation most likely to be retained was vertex/transverse. Apgar scores were lower for retained than unretained second twins. The perinatal mortality rate was significantly higher for the retained second twins (258/1000) compared to the unretained second twins (60.2/1000) (P < 0.002). Early recognition of twin pregnancies in which the 2nd twin is more likely to be retained, and prompt application of appropriate remedial measures would reduce the incidence of retention of the second twin.