The largest IUDs (8 mm diam spirals) caused marked shortening of the postinsertion cycles; the 2 mm spirals caused shortening of some cycles, while others were approximately normal in length; while insertion of threads (0.2 mm diam) resulted in relatively normal cycle lengths.

The normal luteal phase was absent from all postinsertion cycles sampled of ewes with the 8 mm devices and from all but one of the ewes with the 2 mm devices. In contrast, the ewes with the threads had progesterone concentrations within the normal range for the stages of the cycle. All three sizes of IUDs were contraceptive.