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A method of imputing length of gestation on birth certificates.

The length of gestation of a newborn infant is regarded to be the length of time between the 1st day of the last normal menstrual period (LMP) and the day of birth. Attempts to get reliable measures of the gestation period however have been frustrated by the lack of information on some birth certificates regarding the day month or year of LMP. Between 12-16% of all records were missing only the day of LMP. This report examines the feasibility of imputing the length of gestation for records on which the day of LMP is missing but for which the month and year are known; 3 different methods are considered. The study was based on a 20% systematic sample of all 1978 live birth certificates for mothers residing in areas reporting the LMP. Demographic and health characteristics indicated that newborns for whom information on LMP was lacking belonged to a relatively high-risk group with lower than average birth weights. This imprecision is estimated to understate the true proportion of premature babies by about 5%. 1 method of imputing gestation period by assuming the 15th day of the month to be the day of LMP was found to introduce noticeable bias. The random assignment of a day of LMP from 1 to 31 resulted in even greater distortions in period of gestation data. A preferred method making use of the recorded birth weight and month of gestation and which introduces a maximum imputed error of 3 weeks is outlined. The National Center for Health Statistics plans to include this imputation procedure in future revisions of its processing operation. Imputation would then be performed independently for white births black births and births of other races since the relationship between birth weight and gestation is substantially different among these racial groups.