This is up to personal choice. It can be a good idea to get a baseline of what your health is doing. Remember there is a baseline of health you, the mum has to have in order for your body to perceive it safe for you to hold a pregnancy.
Some baseline levels to get checked are: Iron, Ferritin, TSH, Zinc, Vitamin D, Prolactin, DHEAS, Testosterone, 17-OH Progesterone, androstenedione