Homozygous achondroplasia: US distinction between homozygous, heterozygous, and unaffected fetuses in the second trimester.

PURPOSE: To determine if and at what gestational age prenatal ultrasonography (US) enables distinction between homozygous, heterozygous, and unaffected fetuses when both parents have heterozygous achondroplasia.

MATERIALS AND METHODS: Serial US scans of 15 fetuses at 25% risk of homozygous achondroplasia were reviewed retrospectively. Femoral growth curves were established and compared with published standards to determine the gestational age, calculated according to biparietal diameter (BPD), at which femoral length crossed below the 3rd percentile. The presence and severity of achondroplasia were determined clinically after birth.

RESULTS: Femoral length crossed the 3rd percentile at 14.0-16.5 weeks BPD age (mean, 15.6 weeks) in the four homozygous fetuses and at 18.2-26.2 weeks BPD age (mean, 21.5 weeks) in the eight heterozygous fetuses. In the three unaffected fetuses, femoral length did not cross percentiles as gestational age increased.

CONCLUSION: Establishment of a femoral growth curve in the second trimester with serial US scans enables prenatal distinction between homozygous, heterozygous, and unaffected fetuses when both parents have heterozygous achondroplasia.