Both midluteal serum progesterone levels and late luteal endometrial histology should be assessed in all infertile women.

Criteria for diagnosis of luteal phase deficiency vary among specialists. Adequacy of corpus luteum function usually is based on a midluteal serum progesterone (P) concentration or a late luteal biopsy. To compare these two methods of determining luteal phase adequacy, 42 midluteal P levels were compared with late luteal endometrial biopsies from the same cycles in 34 women undergoing evaluation for infertility. Eleven cycles contained both hormonal and histologic evidence of luteal phase inadequacy; 12 contained only hormonal evidence of inadequacy; 9 contained only histologic evidence of inadequacy; and 10 contained both hormonal and histologic evidence of adequate luteal function. These data suggest that both midluteal phase P levels and late luteal endometrial biopsies should be assessed in infertile women, because neither can be used to predict the other.