INTRODUCTION

Individuals choose to use contraception for many reasons:

• All contraceptives provide control over the timing of pregnancy and avoidance of unintended pregnancy
• Condoms provide protection from sexually transmitted infections
• Hormonal contraceptives provide noncontraceptive health benefits (table 1)

A systematic review estimated contraceptive prevalence among women of reproductive age who were married or in a union was 63 percent worldwide and 77 percent in the United States [1]. Nevertheless, unintended pregnancy is a common problem. In a study from the Guttmacher Institute using data from several sources, 49 percent of the 6.7 million pregnancies in the United States in 2006 were unintended [2]. About 5 percent of women of reproductive age had an unintended pregnancy that year, comprising 3.2 million pregnancies. The demographic characteristics of these women are shown in the table (table 2). Forty-three percent of the unintended pregnancies were terminated. These alarmingly high statistics occurred even though most women reported using some form of contraception [3]. The high rate of unintended pregnancy despite contraception highlights the importance of understanding contraceptive efficacy in terms of typical, rather than perfect, use (see 'Effectiveness' below).

Some women choose to not use birth control. The reasons for nonuse were addressed in a report from the Pregnancy Risk Assessment Monitoring System (PRAMS) which surveyed almost 8000 women with a recent unintended viable pregnancy who stated they had not used birth control [4]. The women gave the following reasons for unprotected intercourse: 33 percent felt they could not get pregnant at the time of conception, 30 percent did not really mind if they got pregnant, 22 percent stated their partner did not want to use contraception, 16 percent cited side effects, 10 percent felt they or their partner were sterile, 10 percent cited access problems, and 18 percent selected "other." That one-third of these women did not perceive themselves to be at risk of becoming pregnant speaks to the need for more education.

Unintended pregnancy is associated with significant monetary costs. In 2010 in England, the National Health Service (NHS) provided coverage for 218,100 unintended pregnancies (induced and spontaneous abortions, ectopic pregnancies, births) at a cost of £193,200,000 ($299,200,000) in direct medical costs [5]. Annual medical costs of unintended pregnancy in the United States have been estimated to be $4.6 billion [6]. Contraception is a cost-effective approach for reducing these medical costs [7,8].