Comparison of Gestational Age Classifications: Date of Last Menstrual Period vs. Clinical Estimate

Purpose The purpose was to compare the two different measures of gestational age currently used on birth certificates (the duration of pregnancy based on the date of last menstrual period [LMP] and the clinical estimate [CE] as related to health status indicators. We contrasted these measures by race/ethnicity. Methods NCHS natality files for 2000–2002 were used, selecting cases of single live birth to U.S. resident mothers with both LMP and CE gestational age information. Results Approximately 75% of the records had valid LMP and CE values and for approximately one-half of these, the LMP and CE values did not exactly agree. Overall and for each race and ethnic group, the LMP measures resulted in higher proportions of very preterm, preterm, postterm and SGA births. CE value provided preterm rates of 7.9% and for LMP, 9.9%. The odds ratio of preterm birth for African-Americans using the CE measure was 1.78 [95% Cl 1.77–1.79]. The odds ratio using LMP was 1.93 [95% Cl 1.92–1.94]. Whites were the referent population. Conclusions Different measures of gestational age result in different overall and race-specific rates of very preterm, preterm, postterm, and SGA births. These findings indicate that substituting or combining these measures may have consequences.