Estimating the cost of reproduction is pivotal to understanding the trade-off between current and future reproductive success, a key prediction in life-history theory. Increases in the cost of each reproductive attempt theoretically reduce future reproductive ability. Further, costs may change as individuals grow thus changing the nature of this trade-off. Measuring changes in female locomotor ability during reproduction has been one effective method to measure the cost of reproduction for females. We measured female Diamond-backed Watersnake (Nerodia rhombifer) swimming speed during and after pregnancy to determine if there was a loss of locomotor ability. We then correlated these speeds with measures of reproductive burdening (as estimated by relative clutch mass) and body size to investigate if increased reproductive investment and body size changed locomotor ability and subsequent cost of reproduction. Female snakes swam slower during pregnancy than after. Larger relative clutch masses resulted in slower swimming speeds during pregnancy. Further, shorter individuals showed a greater decrease in swimming speed suggesting a greater cost of reproduction for smaller individuals. Lastly, we demonstrated that additional costs to locomotor ability may be incurred by the female due to weight loss during pregnancy from carrying the burden of reproductive material.