Low birth weight (LBW) is associated with a large number and variety of risk conditions during pregnancy. The number and types of risk conditions per pregnancy were determined in 1,864 white and 872 black mothers delivered at the University of Kansas Medical Center between 1975 and 1978. The incidence of LBW infants increased steadily among white and black mothers as the number of risk factors increased from none to three or four per pregnancy. Among pregnancies without spontaneous premature rupture of membranes (PROM), 51 percent of the LBW infants were born to mothers who had multiple risk factors associated with their pregnancies, even though only 18 percent of these pregnancies were associated with multiple risk factors. Among pregnancies with PROM, 72 percent were associated with multiple risk conditions, and 31 percent resulted in LBW infants. About 90 percent of LBW infants from PROM pregnancies had mothers with multiple risk factors. For all numbers of risk conditions, black mothers had a higher incidence of LBW infants than white mothers. Among black mothers without spontaneous premature rupture of membranes (PROM), the incidence of LBW infants increased from 3.2 percent (10/308) in low (zero)-risk condition pregnancies to 33 percent (16/49) among mothers with three or four risk conditions during the pregnancy. Among white mothers without PROM, the incidence of LBW infants increased from 1.7 percent (12/708) in low (zero)-risk condition pregnancies to 30 percent (19/64) in pregnancies with three or four risk conditions.(ABSTRACT TRUNCATED AT 250 WORDS)