The most appropriate next step in management is to stop the angiotensin receptor blocker (ARB) losartan and monitor the blood pressure in this patient with hypertension who is attempting to conceive. ARBs, ACE inhibitors, and direct renin inhibitors are all associated with adverse fetal and neonatal outcomes; these agents are absolutely contraindicated during pregnancy and should be stopped prior to conception. Physiologic changes associated with pregnancy cause the systemic blood pressure to fall during most of gestation, and there is evidence that neither the patient nor the fetus appears to be at risk from mildly elevated blood pressure during pregnancy. The 2013 American College of Obstetricians and Gynecologists (ACOG) guidelines recommend treating persistent blood pressure elevations of >160/105 mm Hg in pregnant women with chronic hypertension, with a goal blood pressure with medical therapy in these patients being 120-160/80-105 mm Hg. Because this patient's current blood pressure is well below the recommended goal of therapy and her blood pressure would be expected to decline once she conceives, monitoring her blood pressure once the losartan is stopped is a reasonable approach. If her blood pressure rises into the recommended treatment range, reinstitution of treatment with a medication likely to be safe in pregnancy may be warranted.

Labetalol, a β-blocker with some α-blocking effect, may help preserve placental blood flow and is considered to be relatively safe during pregnancy. However, all antihypertensive medications cross the placenta, and it would be preferable to avoid treatment with any medication during early pregnancy. Because this patient might be able to be managed without medication should she become pregnant, switching to another agent from her ARB is not preferable. However, if further medical therapy is required, labetalol would be an appropriate option.