Sonography does appear to be very reliable in confirming ovulation once ovulation has occurred . Disappearance of the follicle is noted in 91 % of cases after ovulation and a decrease in follicle size occurs in another 9% . Other signs suggesting that ovulation has occurred are the appearance of cu;-de-sac fluid , particularly when it was not present in a previous scan , or the development of intrafollicular echoes suggesting the formation of a hemorrhagic corpus luteum .