A new biometric: In utero growth curves for metacarpal and phalangeal lengths reveal an embryonic patterning ratio

Objectives The objectives of this study are to develop gestational age‐specific growth curves for fetal third metacarpal and phalangeal lengths and to determine if fetal hand proportion is established in utero. Methods This prospective cross‐sectional study used 2D ultrasound across gestational ages 12 to 39 weeks to evaluate the third fetal metacarpal and phalangeal measurements. Gestational age‐specific reference growth curves were developed. Associations between continuous variables were assessed using Spearman correlations ( r s ) and restricted cubic splines. A nonlinear biologic regression model was used to predict metacarpal and phalangeal lengths as a function of gestational age. Measurements derived from five cases of thanatophoric dysplasia were used to determine if brachydactyly could be objectified. Results Fetal metacarpal and phalangeal lengths are highly correlated across gestational age ( r s = 0.96, P < 0.001). The mean fetal metacarpal to phalangeal ratio is constant from gestational age 15 to 39 weeks ( r s = −0.07, P = 0.49). Third‐digit metacarpal and phalangeal lengths in thanatophoric dysplasia showed brachydactyly in all cases (5/5), and none of the cases (0/5) demonstrated a normal metacarpal to phalangeal ratio of 0.49. Conclusion We present gestational age‐specific reference growth curves for fetal third metacarpal and phalangeal lengths, which may be used to detect brachydactyly. We demonstrate a prenatal metacarpal to phalangeal ratio of 1:2.