The cost of pregnancy is hard to study in marine mammals, particularly in species that undergo pregnancy while diving continuously at sea such as elephant seals (genusMirounga). We analysed the diving behaviour of confirmed pregnant and non-pregnant northern elephant seals (M. angustirostris,n= 172) and showed that after an initial continuous increase in dive duration, dives of pregnant females become shorter after week 17. The reasons for this reduction in dive duration remain unknown, but we hypothesize that increased fetal demand for oxygen could be the cause. Our findings reveal an opportunity to explore the use of biologging data to investigate pregnancy status of free-ranging marine mammals and factors that could affect pregnancy success.