Objective: To determine the diurnal variation in the onset of the preovulatory LH surge in women.
Design: Prospective open cohort study.
Setting: University hospital research program.
Patient(s): Thirty-five women with infertility resulting from tubal damage that was associated with minor endometriosis or with infertility of prolonged unexplained etiology.
Intervention(s): Women underwent transvaginal ultrasonography and serum E2 estimation daily during monitored cycles before unstimulated natural cycle IVF: exogenous gonadotropins were not administered.
Main outcome measure(s): Serum E2 concentration, follicle diameter, and endometrial thickness.
Results: Of 169 cycles. 155 progressed to an ovulatory LH surge, of which 146 occurred within 8 hours of assessment of the outcome measures. The relationship between follicle diameter and E2 was weak, but an abnormal value for one always was countered by a normal value for the other.
Conclusions: Most women begin the preovulatory LH surge between midnight and 8:00 A.M., but with no particular variation by day of the week. The relationship between follicle size and serum E2 is not sufficiently strong to predict the LH surge confidently on the basis of only one variable, but the LH surge is unlikely to occur before either the follicle diameter has reached 15 mm and/or the serum E2 level has reached 600 pmol/L.