부산대학교 도서관

Previous studies have reported the highest fecundability (monthly probability of conception) in women aged 20 to their early 30s, and declining fecundability with age. To better understand the age pattern of fecundability in Japanese couples we use time to pregnancy (TTP) to estimate conception probabilities by age. A total of 6,752 women aged 20-44 years residing across Japan completed a questionnaire on TTP, number of months between discontinuing contraception and conception for the first child. The analytic sample was n=1,324 women who had discontinued contraception within 60 months before the survey to conceive their first child. Cox proportional hazard regression model was used to estimate fecundability ratios (FR) and 95% confidence intervals (CI) by couple's age. Compared with women aged 24-26 years, women aged 27 or older showed a significantly lower FR. Advanced age of men was also associated with decreased FR. Cumulative probability of conception at 12 months after discontinuing contraception was highest for women aged 24-26 years, i.e. 80% (95% CI: 75%, 84%), and it was lower for higher ages, e.g. 66% (95% CI: 61%, 71%) at ages 30-32 and 48% (95%CI: 39%, 55%) at ages 36-38. Age effects on fecundability were less pronounced for parous compared to nulliparous women, and nulliparous women tended to be older and more likely to seek fertility assistance. Thus it is not just age effects but other, as yet unclear, factors are also likely to be contributing to low fertility in Japan.