At what follicle size should an hcg trigger shot be administered in a medicated cycle?

Depends: Usually ultra sound is by the vaginal route these days. If abdominal ultrasound is performed then the follicles are usually approximately 2 mm smaller. On Clomid (clomiphene) or Femara i usually trigger ovulation with HCG approximately 18 to 20 mm. With gonadotropins i normally trigger ovulation around 20 -21mm. Patients with pcos or polycystic ovarian syndrome triggering ovulation needs to be somewhat later at approximately 21 to 22 mm.