Abstract Background To construct a normal range for the prenatal nasal bone length (NBL) in Brazilians irrespective to the knowledge of the ethnic genetic background. Study design We studied 35 human fetuses (20 males, 15 females) ranging from 14 to 22 weeks of gestation. Gestational age (GA), crown–rump length (CRL), foot length (FL) and body mass (BM) were measured. The X-ray of the head lateral view was made with the specimens placed directly on the film and the NBL was measured. The NBL was correlated with the GA, the CRL, the FL, and the BM using log-transformed data and the allometric model log y = log a + b log x. Results Correlations of the NBL growth with GA, CRL, FL, and BM were positive and significant ( P < 0.05), but NBL vs. BM showed the smallest R indicating this correlation as of little practical use. No sexual dimorphism in the NBL growth in the second trimester fetuses was observed. The NBL grew with positive allometry relative to GA, CRL and BM, but it was allometrically slightly negative relative to the FL in both genders. The NBL be allometrically positive against GA, CRL and BM means the bone grew with growth rates higher than those indices in the period analyzed, but not against FL. Conclusion NBL could be considered an auxiliary measurement in the assessment of the 2nd trimester fetal development because its strong correlation with GA, CRL and FL, even when nothing is known about the ethnicity of the population.