Trying for our last hopefully a girl!!

My husband and I have 4 boys with our youngest turning 6 this month. I have never been on any birth control of any sort ever and we are sexually active. Only till a month ago we are ttc for a girl hopefully and our last. Even though we weren't ttc the past few years, my husband and I are surprised that i haven't gotten pregnant yet. I have been to docs about 3yrs ago after experiencing really painful cramps and had tests done they thought I may have had polycystic ovaries or endometriosis but they could not find anything wrong(and I already have 4kids). After having my fourth child my periods became irregular but have come right again the last 1 1/2yrs. Just wondering if anyone has had something similar happen/happening to them. Maybe my time for conceiving has come to an end?!