First ttc obgyn appointment

Ashley

Hello all! Hubby and I have been ttc for over a year and a half, so I set up a new patient obgyn appointment to talk to my doctor. She first done a pap smear, and done a pelvic exam. She said my ovaries and uterus felt good. But wants me to come back on day three of my period to do blood work to check my hormones. My question is, when checking hormones what is the benefit of waiting until day 3 of my period to draw blood?