The purpose of this study is to learn more about why some people are at greater risk for oral emergency contraceptive failure while others are not. The investigators want to learn if genetic differences impact the risk of emergency contraception failure.

Each of us can respond differently to a drug or medication based on our genetics. An emergency contraceptive, ulipristal acetate or UPA, normally works by stopping or delaying the ovary from releasing an egg (ovulation). Our bodies break down UPA in order to use it through a system call the cytochrome P450 pathway but this pathway can be faster or slower depending on our genetics. The investigators want to learn more about how our individual genetic differences in this pathway change how the ovary responds to UPA. The overall goal of this research is to improve the effectiveness of emergency contraception for all people.

Inclusion Criteria:

• Generally healthy women
• Aged 18-40
• regular menses (every 21-35 days) experiencing ovulatory cycles proven by a single progesterone level of 3 ng/mL or greater during the luteal phase of the screening cycle.

Exclusion Criteria:

• Pregnant, seeking pregnancy, or breastfeeding
• Known allergy to study medication
• Recent use of hormonal contraception
• Irregular periods (<21 days or >35 day cycles)
• Routine use of nonsteroidal anti-inflammatory drugs
• Metabolic disorders
• Smoking
• Any condition that would preclude the provision of informed consent
• Using drugs (within 2 weeks of study enrollment) known to interfere with the metabolism of UPA as well as drugs known to be CYP3A4 inducers, inhibitors, or CYP3A drug substrates