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Utilizing the 1980 Induced Abortion File maintained by the National Center for Health Statistics, we compared gestational age from date of last normal menses and the physician-based estimate of gestational age. An average .51 week difference between the two methods was observed. Beyond seven weeks gestation, the date of last normal menses value was underestimated by the physician-based estimate with a markedly greater divergence after 20 weeks. A relatively greater underestimation of the date of last normal menses interval by the physician estimate was apparent for Whites after 13 weeks. The date of last normal menses value for non-state residents was overestimated across the entire range of the date of last normal menses gestational age distribution until 21 weeks. (Am J Public Health 1989; 79:603-606.) [ABSTRACT FROM AUTHOR]