Ovulation monitoring The women were followed for one cycle by obtaining blood samples and urine samples. The ovulation was monitored with vaginal ultrasonography (aloka-1000, UST-985, 5 MHz transvaginal probe, aloka Co. Ltd, Tokyo, Japan). The thickness of two endometrial layers was measured. The endometrial pattern was assessed according to the classification proposed by Gonen and Casper as follows: an endometrium that was entirely homogeneously hyperechoic without a central echogenic line was considered as a homogeneous echo pattern and triple-line pattern consisted of a central hyperechoic line surrounded by two hypoechoic layers. The growth of follicles, ovulation and the development of corpus luteum were monitored by using two-plane scans. Participants identified the day of the LH surge by testing the morning urine (David ovulation test, Rubio Biotech Co. Ltd., Shantou, China). The day of the LH surge was determined in urine samples when LH surge reached its highest value on which the blood samples were collected as well. The cycle day referred to the first day of menstrual bleeding. Women who have normal ovulatory cycles with intervals of 25-35 days are expected to ovulate on day 11 at the earliest and day 21at the latest. The peripheral blood samples were obtained by vein puncture on the day of the LH surge and 6-7 days after ovulation. Blood was separated by centrifugation within 1 hour, and the serum was stored at -20℃ until the concentrations of LH, FSH, estradiol (E2) and progesterone were assayed.