In Friedman's labour curve, the active phase of labour officially begins at full cervical dilation of 4 cm. However, WHO's 2018 intrapartum care guidelines revised this threshold based on evidence from MANA and other studies. The revised threshold for active phase onset is:

• A 3 cm with regular contractions
• B 5 cm with regular contractions
• C 4 cm with membrane rupture
• D 6 cm with regular contractions ✓

Explanation

WHO's 2018 intrapartum care guidelines revised the definition of active phase onset to 5 cm (and ACOG/SMFM jointly published data supporting 6 cm as the threshold for active labour). The rationale is that cervical dilation before 6 cm follows a slower trajectory than Friedman originally described; labour progress diagnoses of 'failure to progress' made before this threshold often led to unnecessary caesarean sections. Most current guidelines recommend not diagnosing active phase arrest before 6 cm with adequate uterine activity.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.