Talk to Gynecologist on Trying To Conceive

I have one 7 yr old child. Since last 5 months im trying to conceive but get my periods. My menstrual cycle is regular and length is 25 to 27 days. I consulted Dr and she advised me TVS follicular study on 13th day of my cycle. Should i go on 13th day? or earlier like 11th or 12th day because my cycle length is not 28 days. Kindly suggest me.