Most clinics are doing screening at baseline sono or beginning of stimulation. Others are doing it closer to retrieval around stim day 8. It is up to the clinic to decide what is best for them.
Important considerations are that most test will result withing 48-72 hours. Some samples will result as quantity not sufficient or inclusive. Therefore, if it is done at baseline, there will be enough time to repeat the test at no cost to the patient or clinic. If done later in the stimulation, there are increased risks for not resulting in time.