Until what age can a woman postpone the start of childbearing and not experience age-related infertility? This is the question I want to answer in the proposed study. The US had a higher total fertility rate (1.9) than Japan (1.4) in 2001. The very low fertility of Japan, despite a high proportion of births from unintended pregnancy, can perhaps be partly explained by the increase of infertility. Recently more women are experiencing unintended infertility, related in part to delayed marriage and childbearing. However, it is not well understood what is contributing to the current rise of infertility and low fertility in Japan. To my knowledge there have been no epidemiological studies that estimated prevalence of infertility and its risk factors. I hypothesize that both reproductive biology and sociocultural factors are contributing to an increase in infertility and thus to low fertility. While reproductive aging is well characterized in the US, it is not currently well characterized in Japan. Therefore in this proposed study I will compare Japanese vs. US women for behavioral and biological factors that are related to low fertility, which would help Japanese policy makers to construct policies to tackle low fertility that are suitable for the Japanese population. Additionally this proposed study will test whether a biomarker called anti-mullerian hormone (AHM), often used as a marker of ovarian reserve and reproductive aging, can predict infertility in the future. I will estimate the effect of woman's age and other, modifiable factors on risk of infertility with a prospective follow-up design. I will recruit N=1200 women aged 20-44 years old, currently partnered, currently trying to become pregnant, and not on contraception. At intake they will be asked about age, height, weight, smoking, as well as basic demographic characteristics, with a web-based questionnaire. They will answer follow-up questionnaires on menstruation and frequency and timing of sexual intercourse every month until they become clinically pregnant or for up to six months. A subsample of women (N=200) will give blood specimens at intake for AMH measurement, in addition to answering these questionnaires. Those women who did not become clinically pregnant within the six-month follow-up period will be defined as infertile for that observation period. The proposed work will be the first to estimate risk factors for infertility among pregnancy-planners in Japan using a prospective design. The data will show age-related increase of the risk of infertility, which will be very important information for policy makers who try to tackle low fertility in Japan. Additionally, the prospective study will enable us to test the validity of a biomarker to predict infertility in the future. This could help women and couples to decide when to have a baby, which could reduce infertility in couples in Japan and lead to increase in the national fertility level. Until now studies on biobehavioral factors related fertility have been conducted mostly targeting Western populations. The US-Japan comparison in this study will shed light on biobehavioral differences and similarities across populations and their possible impacts on national fertility trends.