Abstract Premature luteinization (PL) refers to a rise in
serum progesterone (P) levels on the day of hCG
administration. Most studies used an absolute P level on
the day of hCG administration as an indicator of PL, and
the cutoff level differed from 0.8 to 2 ng/mL. Some authors
defined PL as a P/E2 ratio of >1. There is a marked
variation in the incidence (13% to 71%), of PL due to
discrepancies in definition, population characteristics and/or
treatment protocols. The pathogenesis of PL in COH is still
poorly understood. Several hypotheses may be considered
to explain this phenomenon: elevation of follicular LH
levels, serum accumulation of HCG from HMG, increased
LH receptor sensitivity of the granulosa cells to FSH, or
poor ovarian response with increased LH sensitivity. The
consequences of this premature elevation of serum P on
IVF outcome remain controversial. Attempts to prevent
COH include: use of Low-dose hCG alone in the late COH
stages, flexible antagonist protocol, use of mifepristone,
aspiration of a single leading follicle, hCG administration
when the levels of serum P exceeded 1.0 ng/mL.