Improving the accuracy of fetal foot length to confirm gestational duration.

OBJECTIVE: To establish normative fetal foot length ranges using last menstrual period (LMP) and ultrasound dating by biparietal diameter and to examine variations in these ranges by ethnicity.
METHODS: A consecutive series of 1,099 eligible subjects receiving abortions had fetal foot lengths measured directly. Models of fetal foot length were developed by using assessment of gestational duration by LMP alone, ultrasonography alone, and "best estimate" (LMP confirmed by ultrasonography).
RESULTS: The full sample model using ultrasound dating (n = 1,099) yielded the following equation: foot length = -30.3 + days of gestation x 0.458 (R(2) of 0.92). Regression by LMP-determined gestational duration by using the "best estimate" sample (n = 491) provided an almost identical equation (foot length = -29.8 + days of gestation x 0.45) and a similar R(2) value of 0.87, although the standard errors were larger. Gestational duration by ultrasonography alone produced a better model fit than duration by LMP alone. Regressions by ethnicity were not significantly different compared with the simple regression, regardless of method used to determine gestational duration.
CONCLUSION: A reconsideration of fetal foot length measurements to confirm gestational duration is important. More accurate tables of these measurements allow for greater precision in correlating gestational duration and foot length. Fetal foot length tables using ultrasonographically confirmed gestational duration and current statistical standards should replace tables currently used. Biparietal diameter as a single measurement provides adequate estimation of gestational duration in the second trimester for pregnancy termination, proving more reliable than LMP dating. LEVEL OF EVIDENCE: II-2.