Fecundity is defined as the biological capacity to reproduce, whereas fertility is defined by demographers as the production of a live birth (Pressat, 1985; Wood 1994 p.3).
Fecundity cannot be directly measured, so I use time to pregnancy (TTP) as a measure of fecundity. Couples with shorter TTPs are expected to have higher fecundity, and those with longer TTPs to have lower fecundity.
From studies that measured TTP of couples trying to conceive, it is empirically known that there are couples with higher and lower fecundity. I am interested in behavioral and biological factors that can explain the heterogeneity of human fecundity within and across populations.
Obviously TTP is associated with coital frequency, an important behavioral factor. Age is an important and established biological factor that is associated with TTP. However, even after adjusting for age and coital frequency, there still exists heterogeneity of fecundity across couples. I am studying what other factors can explain this diversity in biological ability to produce a child. In our ongoing collaborative project (Interdisciplinary Investigation on Technology, Environment, and Fertility, IITEF study), we are going to investigate how technology and environmental chemicals might affect human fecundity through changes in sexual norm and behavior and altered probability of conception.
2. Study on time to pregnancy of Japanese couples trying to conceive their first child
We have conduced a couple of epidemiological studies in Japan to estimate TTP. One of our papers (Konishi et al. 2018) describes the age pattern of TTP in Japanese couples trying to conceive their first child. The estimated TTP tended to be longer in the Japanese couples compared to the reported values in Western settings. Our ongoing project (Interdisciplinary Investigation on Technology, Environment, and Fertility, IITEF study) is aiming to examine HOW and WHY fecundity of Japanese couples seems to be lower compared to their Western counterparts.