Since twin pregnancies often result in poor perinatal outcomes, many physicians advise prolonged bed rest. Recommendations concerning the timing of bed rest conflict and are made with little assessment of costs. This review of twin pregnancies in the North Central Illinois perinatal region establishes that twins are most vulnerable if they are born between 27 and 34 weeks' gestation. If bed rest is to be imposed, it probably should be timed so as to influence this vulnerable period. Intervention (bed rest in the hospital from 27 to 34 weeks' gestation) would cost between $5,720 and $6,909 per twin pregnancy, whereas nonintervention (intensive care nursery charges for infants born before 34 weeks' completed gestation) would cost $1,689 per twin pregnancy. Before intervention can be universally advocated and costs of this magnitude incurred, a prospective controlled trial to determine the efficacy of bed rest in twin pregnancy is mandatory. © 1979.