Female coypus Myocastor coypus in captivity produce more litters than expected by chance following days when they are undisturbed and less after they are disturbed. The observed distribution can be explained using simulations which assume that females can delay parturition for up to three days, depending on the level of disturbance on a previous day. The advantage of delay for wild coypus may be that parturient females can delay parturition if detected by a predator and move to a safer location. The main cost of delay is an increase in the average proportion of young that die in the last few days of pregnancy and in the days following parturition. These costs and benefits may be similar to those involved in the evolution of gestation periods.