In oviparous species, preovulatory maternal effects such as investment in yolk quantity and composition or levels of yolk steroid hormones might potentially affect hatchling body sizes, growth rates, performance, and sex in species with temperature-dependent sex determination. We investigated the effects of egg mass and levels of testosterone and 17β-estradiol in the yolk on the phenotypes of hatchling Magdalena River Turtles (Podocnemis lewyana) in three populations in the Magdalena River drainage of northern Colombia. We hypothesized that, under homogeneous incubation conditions, the differences documented among clutches would be attributable to familial effects, caused by either heritable genetic factors or preovulatory maternal effects. Eggs were incubated artificially at the pivotal temperature for the species (33.4°C) and hatchlings were reared in the laboratory for 2 mo. Two eggs from each clutch were tested for testosterone and 17β-estradiol levels. Sex ratios, hatchling size, mass, and righting times varied both between clutches within a site and between populations. Egg mass was positively related to hatchling body size and mass, both at hatching and at 2 mo of age. Levels of 17β-estradiol concentrations influenced hatching success rates and incubation periods. Sex ratios were marginally related to 17β-estradiol levels, with clutches and sites with higher levels tending to produce fewer males. Our results provide evidence that preovulatory maternal effects play a role in influencing phenotypic attributes related to hatchling survivorship such as size, growth rate, and performance, and also seem to interact with incubation temperature to determine the sex of each individual and the sex ratio of the clutch, which has implications for both parental and offspring fitness.