The tendency to repeat gestational age and birth weight in successive births, related to perinatal survival.

The risk of perinatal death was studied in relation to the tendency for mothers to carry their fetuses to similar gestational ages and birth weights in successive pregnancies. The data for this study are derived from all 635 140 births in Norway during a 10-year period (1967-76). The findings demonstrate that those babies that were born similar in age and size to their elder siblings ran the lowest risk of perinatal death. For example, the perinatal mortality rate (PMR) for second-birth babies weighing between 2 501 and 3 000 g is 9.1 per 1 000 births if the first baby weighed 3 000 grams or less. But the PMR is 50% higher, 13.3 per 1 000 births, when the first baby weighed between 3 001 and 3 500 g. If the first baby weighed more than 3 500 g, then the PMR is 3 1/2 times greater, 32.1 per 1 000 births. In addition to gestational age and birth weight, the Medical Birth Registry of Norway also records measurements of crown-heel length. The length data show that babies at greatest risk for perinatal mortality tend to be relative long and thin rather than symmetrically developed. For example, the PMR for second babies weighing between 2 501 and 3 000 grams who were disproportionately long (greater than 46 cm) was 4.6 times greater than the PMR for the average length babies of the same birth weight. The investigation of cause-specific mortality rates showed that most categories were increased for the higher risk babies. However, congenital malformation was the principal contributor to deaths of the relatively short babies.(ABSTRACT TRUNCATED AT 250 WORDS)