Mifepristone Dose for Induction of Labour
For induction of labour at term with a live fetus, administer mifepristone 200 mg orally as a single dose, with a possible second 200 mg dose 24 hours later if labour has not commenced. This dosing regimen is supported by the highest quality randomized controlled trial evidence and represents the lowest effective dose for cervical ripening and labour induction 1, 2, 3.
Evidence-Based Dosing Protocol
Single dose of 200 mg mifepristone appears to be the lowest effective dose for cervical ripening, with demonstrated efficacy in increasing the likelihood of cervical ripening at 72 hours (RR 2.13,95% CI 1.15 to 3.97) 2.
The recommended protocol is:
- Day 1: Administer 200 mg mifepristone orally 1, 3
- Day 2: If labour has not commenced, administer second 200 mg dose 1, 3
- Day 3-4: Observe for spontaneous labour or proceed with conventional induction methods if needed 1, 3
Clinical Efficacy Outcomes
Women receiving mifepristone 200 mg demonstrate significantly superior outcomes compared to placebo:
- Spontaneous labour within 48 hours: Mifepristone-treated women are 2.41 times more likely to be in labour or have a favourable cervix (RR 2.41,95% CI 1.70 to 3.42) 2
- Spontaneous labour within 96 hours: Effect persists with RR 3.40 (95% CI 1.96 to 5.92) 2
- Caesarean section rate: Reduced by 26% (RR 0.74,95% CI 0.60 to 0.92) 2
- Failed induction: Significantly reduced (RR 0.40,95% CI 0.20 to 0.80) 2
- Mean Bishop score gain at 48 hours: 2.58±1.33 versus 1.15±0.97 in expectant management (p<0.001) 3
Maternal Pharmacokinetics and Safety
Maternal serum mifepristone peak levels range from 200 to 700 µg/L with a half-life of 21.7 hours 4. The concentration in umbilical cord blood remains low and stable, with an umbilical/maternal ratio of 0.25±0.08, indicating limited fetal exposure 4.
Side effects associated with mifepristone at this dose are mild 4. However, abnormal fetal heart rate patterns are more common after mifepristone treatment (RR 1.85,95% CI 1.17 to 2.93), though there is no evidence of differences in other neonatal outcomes 2.
Oxytocin Requirements
Women who deliver vaginally after mifepristone pretreatment require significantly less oxytocin, and only 22.58% undergo spontaneous delivery after mifepristone treatment compared to 4.84% of controls (p<0.01) 4. Additionally, mifepristone-treated women are less likely to need augmentation with oxytocin (RR 0.80,95% CI 0.66 to 0.97) 2.
Alternative Dosing Regimens
A higher dose regimen of 50 mg every 12 hours for 2 days (total 200 mg) has also been studied, showing significant increases in cervical Bishop score and higher spontaneous delivery rates (22.58% versus 4.84%, p<0.01) 4. However, the single 200 mg dose appears to be the most practical and effective approach 2.
Critical Contraindications and Limitations
There is insufficient information on the occurrence of uterine rupture/dehiscence in women with previous caesarean section receiving mifepristone for labour induction 2. Given that mifepristone is mentioned in the context of medical abortion with a dose of 600 mg in patients with hereditary angioedema 5, extreme caution is warranted in any patient with uterine scarring.
Do not confuse this indication with medical abortion, where mifepristone 600 mg is used in combination with gemeprost pessary 5. The labour induction dose is substantially lower at 200 mg 1, 2, 3.
Common Pitfalls to Avoid
- Do not exceed 200 mg per dose - higher doses have not been shown to improve efficacy and may increase adverse effects 2
- Monitor for abnormal fetal heart rate patterns - these occur more frequently with mifepristone (RR 1.85) 2
- Be prepared for instrumental delivery - mifepristone-treated women are more likely to require instrumental delivery (RR 1.43,95% CI 1.04 to 1.96) 2
- Anticipate more painful uterine contractions and a trend toward higher rates of cephalopelvic disproportion, which may be directly related to mifepristone action 3
- Insufficient evidence exists to support routine use over standard cervical ripening agents currently in practice 2
Timing Considerations
The mean time interval between day 1 of treatment and onset of labour is significantly shorter in the mifepristone group, with an enrollment-to-delivery interval of 2.69±2.06 days versus 3.77±1.86 days in expectant management (p<0.001) 3.