Coadministration of ethinyl estradiol (50 µg single dose) and ritonavir (500 mg twice daily) decrease ethinyl estradiol AUC and Cmin by 40% and 32%, respectively. Due to reductions in ethinyl estradiol concentrations, barrier or other non-hormonal methods of contraception should be considered with concomitant ritonavir use when dosed as an antiretroviral agent or as a pharmacokinetic enhancer. Ritonavir is likely to change the uterine bleeding profile and reduce the effectiveness of estradiol-containing contraceptives.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (500 mg every 12 hours for 16 days) with a single dose of ethinylestradiol (50 µg), in 23 subjects, caused a 40% and 32% decrease in ethinylestradiol AUC and Cmax, respectively. Alternate methods of contraception should be considered.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Coadministration of ritonavir (500 mg 12 hourly) and a single dose of ethinylestradiol (50 µg in a fixed combination oral contraceptive) was studied in 23 healthy volunteers. There was a 40% decrease ethinylestradiol AUC and a 32% decrease in Cmax. Concentrations of ritonavir remained above the protein binding corrected IC90 for ritonavir. Dosage increase or alternate contraceptive measures should be considered.
Effect of ritonavir on the pharmacokinetics of ethinyloestradiol in healthy female volunteers. Ouellet D, Hsu A, Qian J, et al. Br J Clin Pharmacol, 1998, 46:111–6.