Perineal length: norms in gravid women in the first stage of labour.

Introduction: The purpose of this study was to generate normative data for perineal length for Caucasian and Asian women in labour.

Methods: The distance from the posterior fourchette to the centre of the anal orifice was measured in 1,000 women in the first stage of labour. Data on ethnicity, body mass index, delivery mode and perineal trauma were collected prospectively.

Results: The mean perineal length in Caucasian women was 3.7 +/- 0.9 cm and in Asian women, 3.6 +/- 0.9 cm. Primigravid women with short perineum were more likely to have a third-degree perineal tear in labour (p = 0.03).

Conclusion: This is the first paper to report normative data for perineal length in Caucasian and Asian women in labour. We found a negative correlation between perineal length and third-degree tear in primigravid women. These data may be useful in clinical practice to determine the risk of significant perineal tears in labour.