Ttc w/baby #4

Mrs • I`m 33 w/3 daughters & a 1 son.. I have been w/my husband 15 yrs, but only been married 5 yrs

What am I doing wrong? I have 3 kids 12, 6, & 24 months. I have been off my bc for almost a year & still haven’t conceived. Do I need to contact my dr? Or is this normal?