Ans. c. Occurs after follicular ripening by FSH (Ref: Williams Obstetrics 23/e p355-356: Ganong 22/e p438: Shaw's 13/e p212)Follicular phase is under control of FSH and LH. Both FSH and LH is responsible for follicular ripening, but a surge in LH secretion triggers ovulation.OvulationLH surge is primarily caused by 'positive feedback' from sustained levels of estrogen.A surge in LH secretion triggers ovulation.A surge in LH secretion triggers ovulation and ovulation normally occurs about 9 hours after the peak of LH surge at midcycle.Tests of OvulationBasal body temperature:BBT falls at the time of ovulation by about 1/2 degree FahrenheitQ.Subsequently, during the progestational half of the cycle the temperature is slightly raised above the preovulatory level, and the rise is order of 1/2 to 1 degreeQ.This phenomenon is due to the thermogenic action of progesteroneQ, and is therefore presumptive evidence of presence of functioning corpus luteum and hence ovulation.Endometrial BiopsyDone preferably one or two days before the onset of menstruationQSecretary changes prove that the cycle has been ovulatoryQEndometrium should be subjected to guinea pig inoculation and culture to rule out genital tuberculosis which is present in 5-10 % of Indian women complaining of sterilityToday, endometrial biopsy is omitted as a routine investigation or infertility and ovulation best monitored by serial ultrasound scanning. Endometrial biopsy is taken only in suspected tubercular endometritisFern TestCervical mucus, when viewed under low power microscope it shows during the oestrogenic phase, a characteristic pattern of fern formationQ.This feming disappears after ovulation, and if previously present its disappearance is presumptive evidence of corpus luteum activity.The ferning is due to the presence of sodium chloride in the mucus secreted under estrogen effectQOvulation mucus has the property of great elasticity and will withstand stretching upto 10 cm, phenomenon is k/a Spinnbarkeit or thread testQ for estrogen activity.UltrasoundUltrasound has now become the standard procedure for monitoring maturation of the graffian follicle, and in detecting imminent ovulation in in-vitro fertilization and timing intercourse.This requires daily ultrasonic visualization of ovaries from 10th to 16th day of menstrual cycle.