Abstract The length of pregnancy is a major determinant of newborn health and varies due to both environmental and genetic influences, yet their combined effects are not fully understood. The purpose of this thesis was to examine whether the time interval between pregnancies and a history of miscarriage alter maternal genetic influences on the timing of delivery. The study analysed pregnancies from women with more than one previous birth for the interpregnancy interval models and first pregnancies for the miscarriage models, using full-genome genotype information and polygenic scores that summarise the inherited contribution to pregnancy duration. Statistical evaluation was performed with linear regression and genome-wide tests of gene–environment interaction. The results showed that maternal genetic effects on gestational duration did not differ substantially across interpregnancy interval categories, although genetic influences were somewhat stronger at intermediate intervals. A notable interaction was detected at a genetic region near the gene ATRNL1, where the effect of the maternal allele was stronger among women with a previous miscarriage, and additional evidence pointed to a miscarriage- associated pattern of genetic variation at the DPYSL3 locus. These findings indicate that maternal reproductive history can modify the expression of specific genetic factors that contribute to pregnancy timing and underline the need for future research integrating molecular data from relevant reproductive tissues. Keywords: gestational duration; interpregnancy interval; miscarriage; genome-wide association study; ATRNL1; DPYSL3