The purpose of the study was to examine whether coresidence with parents affects the reproductive success of daughters in modern Japanese society. In Study 1, I tested whether women who were living with parents at the time of marriage would experience earlier first childbirth. In Study 2, I tested whether women who were living with parents when their firstborn child was young would experience earlier second childbirth. Cox regression models were used to estimate the relative risk of reproduction, taking into account several potential confounders. The results indicated that parents-in-law, especially mothers-in-law (i.e., the husband's mother), exerted the strong positive effects on the reproduction of daughters. These findings imply that, in a traditionally patrilocal country such as Japan, the relationship between mother- and daughter-in-law influences reproductive success more than that between biological mother and daughter. It is thus necessary to consider cultural context when testing the impact of grandparental investment in modern society.