Dating fetal gestational age in isolated short femur using pars basilaris biometry: A retrospective CT study.

OBJECTIVES

To compare gestational-age estimation from pars basilaris maximal width (MW) and maximal length (ML) and from femur length (FL) measured on fetal CT, against first-trimester crown-rump length (CRL) as reference, in fetuses with isolated short femur; and to assess the frequency and correlates of "decoupling" (discordance between biometric growth and physiological maturation).

MATERIALS AND

METHODS

We retrospectively reviewed all fetal CT scans performed between January 1, 2009 and May 31, 2023 in the public hospitals of Marseille for isolated short femur detected on antenatal ultrasonography. CT examinations were re-read, a standardized measurement protocol was defined, and intra- and inter-observer reproducibility were assessed.

RESULTS

One hundred twenty-nine fetal CTs were included. Median maternal age was 30 years, and median gestational age at CT was 30 weeks. In decoupling situations, ML (95% confidence interval [CI], 29.3-31.1 weeks) and MW (95% CI, 29.3-31.0 weeks) dated gestation more accurately than FL (95% CI, 28.0-30.0 weeks). Pearson correlation coefficients with CRL-based gestational age were 0.867 for the mean of ML and MW, 0.856 for MW alone, and 0.851 for ML alone; FL showed the lowest correlation (0.798; all P<0.001).

CONCLUSION

Pars basilaris-based dating (MW/ML) appears robust and clinically relevant in fetuses with isolated short femur, particularly when decoupling from FL is present.