Mortality in premature mice at birth and during neonatal development.

This paper reports a statistical study of mortality in about 2 500 premature mice at birth and during neonatal development. The premature mice were removed by caesarean section and reanimated for 30 min.; surviving mice as well as newborns delivered vaginally on day 20 of pregnancy (control mice) were raised from birth to day 20 by nursing mothers spontaneously delivered at the same time. The mortality rate in the premature mice and control newborns was estimated immediately after birth by caesarean section and 30 min. after reanimation or parturition, 6 h after birth and at 9 h every day from neonatal days 2 to 20. The caesarean section did not cause mortality. The chances of the survival of premature mice at birth depended on both gestational age and birth weight. Premature mice could not survive if their gestational age was less than 19 days and their birth weight less than 33% of that of newborn mice at term. There was no relationship between mortality and the number of newborns per litter. Two types of mortality were observed in premature mice, mortality at reanimation and breeding mortality. Mortality at reanimation was about 13%; breeding mortality, occurring only during the first 3 neonatal days, was of two types: primary neonatal mortality (10.9%), occurring from the end of reanimation up to 6 h after birth, and secondary neonatal mortality (6.9%) which appeared from days 2 to 3. The role of some factors causing mortality has been discussed.