The Northwestern University multihospital twin study. I. A description of 588 twin pregnancies and associated pregnancy loss, 1971 to 1975.

Five hundred eighty-eight women who were delivered of twins at 13 hospitals affiliated with the Northwestern University Medical School or its outreach institutions between 1971 and 1975 were studied. Eighty-six of the infants did not survive the pregnancy, delivery, or the first week post partum. Maternal demographic and obstetric characteristics are outlined. Infants are categorized by birth order and by birth weight. The mean weight (and standard deviation) of first twins was 2,388 gm (+/- 742 gm) and that of second twins was 2.314 gm (+/- 766 gm). The rate of pregnancy wastage was highest in mothers who were under 20 years of age, who were of low parity, and who were delivered prior to the physiologic maturation of the fetus. The crude mortality rate of twin 1 was 6.1/100 versus 8.5/100 for twin 2. The causes of death in all infants over 2,000 gm are listed. All pregnancy losses are categorized into unavoidable deaths (26.6%), probably not avoidable deaths (59.4%), and possibly avoidable deaths (13.9%).