Significant difference in the frequency of out-of-phase endometrial biopsies depending on the use of the Novak curette or the flexible polypropylene endometrial biopsy cannula ('Pipelle').

A statistically significant difference in the frequency of out-of-phase biopsies was found on comparison of day dating in two sequential series of late luteal endometrial biopsies taken with the standard 4-mm stainless steel Novak curette and a polypropylene cannula ('Pipelle'). Since the only variable in these two series was the type of biopsy instrument used, it is suggested that the diagnosis of inadequate luteal phase can be affected by an extrinsic non-biological factor, i.e., the type of biopsy instrument used. This technical 'artifact' deserves further study.