The anogenital ratio can be used to screen newborn female infants for fetal virilization with androgen-induced labioscrotal fusion. It applies to both premature and full-term female infants and is independent of body size.
Measurements:
(1) distance from anus to the base of the clitoris
(2) distance from anus to the fourchette
(3) distance from the fourchette to the base of the clitoris
anogenital ratio =
= (distance from anus to fourchette) / (distance from anus to base of clitoris)
Interpretation:
• The mean ratio for newborn infants is 0.37 with +/- 0.07
• A ratio > 0.50 exceeds the 95% confidence limits. This suggests labioscrotal fusion and indicates the need for further evaluation.
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