Regression equations for naris to midtracheal distance in mm derived from gestational age (Gest), body weight (Wt), Occipitofrontal head circumference (OFC), crown-rump length (CRL), shoulder to umbilicus length (SUL), and foot length (FL)
| Measurement | Regression equation | Residual SD |
| Gest (weeks) | 25.7 + 1.97 Gest | 8.89 |
| Wt (g) | 64.3 + 0.01 Wt | 7.80 |
| OFC (mm) | 16.7 + 0.26 OFC | 8.67 |
| CRL (mm) | 2.20 + 0.30 CRL | 6.56 |
| SUL (mm) | 23.4 + 0.49 SUL | 8.39 |
| FL (mm) | 24.4 + 1.06 FL | 6.42 |