What's the difference of Clomid 3-7 and 5-9?

Recruitment of follicles during the cycle starts with the menses. The idea behind starting stimulation earlier is to capture more of the available follicles for recruitment increasing the likelihood of multiple eggs maturing during stimulation. The hope is that more eggs are matured and therefor the likelihood of getting one fertilized is improved and perhaps this then increases the pregnancy rate for the cycle.