Serum levels of placental protein 14 do not accurately reflect histologic maturation of the endometrium

Objective: To determine whether serum levels of placental protein 14, a major product of the progesterone-induced secretory endometrium, accurately reflect histologic maturation of the endometrium. Methods: Daily serum levels of placental protein 14 were compared in 50 normally cycling women with normal or delayed endometrial maturation, as assessed by histologic dating of an endometrial biopsy in the midluteal phase of the same cycle. Ten of these subjects had placental protein 14 measurements but no biopsy in an additional cycle to examine the potential effects of the biopsy on secretion of this protein. Results: Serum placental protein 14 concentrations started to increase 8 days after the LH surge and peaked at similar levels on the first day of the next menses in biopsy and non-biopsy cycles. The biopsy cycles had a shorter luteal phase but a slightly faster increase in placental protein 14 concentrations. Both the integrated secretion of this protein and single measurements on the day of the biopsy or at the onset of the next menses overlapped substantially in women with different degrees of endometrial development, even when differentiation of the endometrium was severely delayed. Conclusion: Serum measurements of placental protein 14 do not accurately predict, and thus should not replace, histologic evaluation of the endometrium at nidation.