This study describes the evolution in fetal and neonatal mortality rates among twin pairs born in 22 hospitals located in the eastern regions of the province of Quebec in 1976-1978 (n = 776 pairs) and 1982-1985 (n = 712 pairs). It also assesses the contribution of maternal factors, obstetrical care and characteristics of twins in the variation of the risk of death over time. The fetal mortality rate did not improve from 1976-1978 (22.6 per 1000) to 1982-1985 (28.1 per 1000). However, the neonatal mortality rate declined from 44.7 to 34.7 per 1000 liveborn first twins and from 56.8 to 36.1 per 1000 liveborn second twins. For first twins as for second twins, birthweight-specific neonatal mortality rates decreased within birth weight categories under 2500 g. In the second period, 96.9% of twin pregnancies were detected before confinement compared to 59.6% in the earlier period. The proportion of twins delivered by obstetricians, the percentage of twin births occurring in ultraspecialized perinatal units and the frequency of caesarean sections increased markedly. The proportion of preterm births increased over time (34.5% vs 43.1%) whereas the percentage of low birthweight twins decreased but not significantly (54.3% 51.6%). In this study, changes in maternal age, parity, educational level, sex of pairs, qualification of the physician, and level of care available at the hospital of birth, did not account for the decrease in neonatal mortality rates among twins. The increase in the frequency of caesarean sections seemed to explain only a small proportion of the decrease in the neonatal mortality rate among second twins. In the second as well as in the first period, the neonatal mortality rate for twins was six times higher than that for singletons.

Four hundred and thirty four twins occurring in 220 women were studied during a period of 11 years (1969–1979) at Notre-Dame Hospital.

Perinatal mortality (< 28 days) was compared before and after 1974, and the impact of ultrasound technique upon perinatal outcome was assessed during the second period (1974–1979). The main factor associated with perinatal mortality was low birth weight caused by either prematurity or intrauterine growth retardation.

While fetal mortality remained unchanged within the two study periods, neonatal mortality decreased from 68.2/1,000 to 28.9/1,000 mainly due to increased survival rate of twins below 1,500 g at birth.

With identical perinatal care during the same period, perinatal mortality and incidence of intrauterine growth-retarded twins remained unchanged despite early diagnosis by ultrasound.