Tumors vary in how they affect pregnancy depending upon the hormone secreted. Some hormone oversecretion syndromes must be controlled to allow pregnancy to proceed without undue maternal and fetal morbidity (Cushing's disease and hyperthyroidism) whereas treatment during pregnancy for other tumors is not necessary. Surveillance for tumor growth during pregnancy is necessary primarily for prolactinomas. A literature search was conducted to identify the effects of pregnancy on pre-existing pituitary tumors and the effects on the outcome of pregnancy due to hormone oversecretion by pituitary tumors. Results show that hyperprolactinemia and Cushing's disease may interfere with fertility and usually need to be controlled to allow for conception. Cushing's syndrome, acromegaly and hyperthyroidism secondary to hypersecretion of thyroid-stimulating hormone (TSH) may increase maternal morbidity (gestational diabetes, hypertension) and fetal morbidity and mortality. Intervention is warranted to remove a tumor that secretes adrenocorticotropic hormone (ACTH) during pregnancy to reduce the risk of fetal loss and to control hyperthyroidism. In contrast, surgery or medical therapy for adenomas that secrete growth hormone (GH) and for clinically nonfunctioning adenomas is not indicated during pregnancy. Pregnancy may cause an increase in the size of tumors that secrete prolactin (PRL), especially macroadenomas, so close surveillance is indicated and re-institution of bromocriptine therapy may be necessary to treat such an increase in tumor size. An increase in the size of other types of tumors during pregnancy is very rare.