Interbirth intervals limit maternal fecundity, but cannot be shortened over a certain treshold without risking harmful consequences, both for mother and offspring. However, different socioecological conditions may change the individual effects of the length of the interbirth-interval. In this work, I analyzed historical data from the Krummhörn population (18th-19th Century, Germany) to estimate, if the maternal and paternal grandmother could moderate the effect of the interbirth interval on the affected child. Several generalized linear models either with or without mixed effects have been applied using regression analysis. Results indicate, that only the maternal grandmother, but not the paternal grandmother, weakens the increase in the early mortality-risk, which may result from a shortened interbirth interval. Regarding one year old children, sexspecific effects corresponding to the grandmaternal X-chromosome relatedness have been found, between the presence of the paternal grandmother and the length of the following interbirth interval. Longterm consequences on the marriage probability seem to depend on the intrafamiliar sibship configuration. Results are discussed in context of the grandmother hypothesis and local resource competition.