In 1989 the LA Times quoted a study reporting that the survival rate of premature infants had not improved over the last seven years. The study was published in the New England Journal of Medicine by Maureen Hack and Avroy Fanaroff.

Maureen Hack and Avroy A. Fanaroff published a follow-up study in 2003 showing a dramatic improvement in premature infant survival rate between the years 1993 and 2000. Semin Perinatol. 2003 Aug;27(4):281-7. doi: 10.1016/s0146-0005(03)00055-7.

Detailed outcomes for infants with birth weights between 501 and 800 g, and gestational ages of 23 to 25 weeks are also presented because they dramatically document the changes over time. Mortality for the entire cohort decreased from 23% in 1987/1988 to 17% in 1993/1994 and 14% in 1999/2000.

The percentages of low birth weight babies by state since 2005 are available from the CDC.

Infant mortality rates (with some by state) 1995 to 2013 from CDC.

Data by state back to 1995 is available from the CDC Wonder database, which also has compilations of the causes of infant mortality in the U.S.