Jamieson, Fausset and Brown on Exodus 1:16

16. if it be a son, then ye shall kill him--Opinions are divided, however, what was the method of destruction which the king did recommend. Some think that the "stools" were low seats on which these obstetric practitioners sat by the bedside of the Hebrew women; and that, as they might easily discover the sex, so, whenever a boy appeared, they were to strangle it, unknown to its parents; while others are of opinion that the "stools" were stone troughs, by the river side--into which, when the infants were washed, they were to be, as it were, accidentally dropped.