Researchers conducted this study to compare women who enroll in EC trials to those who decline and to understand why eligible women refuse to participate.

Data were collected from all women seeking EC at three clinics in the USA and UK for nearly one year. The primary outcome measures were pregnancy risk calculated by adjusted cycle day of ovulation.

Enrolled and non-enrolled women had similar mean ages and equal mean cycle lengths. However, the enrolled and non-enrolled groups were different regarding the adjusted cycle day of UPSI, the regularity of their cycles, recent hormone use, breastfeeding, and the number of other acts of UPSI they had engaged in during the same process, and their willingness to participate in the study. 

The study conducted that women in EC trials are likely to face higher pregnancy risk than the general population. Clinical trials might overestimate the number of pregnancies averted by treatment because the number of expected pregnancies in trial populations is not representative of the people of all EC seekers. This information could be useful in protecting the public health impact of expanded EC access.