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 (rs ) 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 (rs  = 0.96, P < 0.001). The mean fetal metacarpal to phalangeal ratio is constant from gestational age 15 to 39 weeks (rs  = -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.