The age of a blow fly larval specimen found on a body can be used to estimate a minimum postmortem interval by comparing a physical feature of a larva (e.g., length) to a reference growth curve created under similar conditions. A better understanding of factors known to influence growth rates would lead to more precise estimates of larval age. A factor known to influence insect development, but almost always lacking in blow fly larval growth curves, is sex. We wanted to understand how sex might affect larval growth rates and therefore lead to more precise age estimates, in the forensically important blow fly Chrysomya megacephala (F.). We examined sex for an entire age cohort at 70 h and found on average, males were longer than females. To assess whether this difference would cause an investigator to interpret the results differently if comparing a larval specimen of one sex based on reference data from the other sex, we made sex-specific 95% prediction intervals. We did not find a difference when comparing a male specimen with the female prediction interval, but did find more females outside the interval than expected using the male interval. All females outside the male interval were shorter. Investigators commonly remove the largest individuals from a body to estimate age, so these females are less likely to be chosen. These results do not support the generation of sex-specific growth curves for this species, though this study's narrow scope means more information is required before making a final conclusion.