Evidence from new study supports the existing FSRH recommendation that commencement of hormonal contraception should be delayed for 5 days after UPA-EC.
The mechanism of action of ulipristal acetate 30mg for emergency contraception (UPA-EC) is to delay ovulation for at least 5 days until sperm from previous unprotected intercourse are no longer viable.
A pharmacodynamic study  published in Contraception studied 33 healthy women (aged 18-35, BMI ≤28 kg/m2) with proven ovulation for three cycles. In the first cycle, the women received ulipristal acetate (UPA) 30mg when they had a dominant follicle of 15mm; they then had ultrasound and blood test monitoring until there was evidence of follicle rupture, or for 7 days. Cycle 2 was a washout cycle. In cycle 3, the women again received UPA 30mg when they had a 15mm dominant follicle, but this time started a 30mcg EE/LNG combined oral contraceptive (COC) two days after the UPA.
Results: Follicular rupture was significantly (p=0.008) more likely to occur in the 5 days after administration of UPA in cycle 3 [9/33 (27%)] than in cycle 1[1/33 (3%)].
Conclusion: COC taken two days after UPA 30mg significantly reduces the ability of the UPA to delay ovulation for at least 5 days.
This new evidence supports the existing FSRH recommendation that commencement of hormonal contraception should be delayed for 5 days after UPA-EC.
1. Edelman AB, Jensen JT, McCrimmon S, Messerle-Forbes M, O'Donnell A, Hennebold J Combined oral contraceptive interference with the ability of ulipristal acetate to delay ovulation: A prospective cohort study. Contraception. 2018 Aug 14. pii: S0010-7824(18)30396-2. DOI: https://doi.org/10.1016/j.contraception.2018.08.003 [Epub ahead of print]