Low Blood Pressure
A literature search of MedLine, the Internet , and major medical texts found no lower limit for normal blood pressure in adults without symptoms.
Low blood pressure associated with symptoms such as dizziness, heart palpitations, shortness of breath, or chest pain should be brought to the attention of your health care provider . Low blood pressure in association with any of the above symptoms may be caused by chronic fatigue syndrome, dehydration, embolism, endocrine disorders, excessive blood loss, heart disease, heat stroke, medications, neurologic disease, or severe infection.
A lowering of blood pressure during pregnancy is normal.
As early as the 7th week  of pregnancy blood pressure begins to decrease reaching its lowest level between the 18th  to 28th week. Thereafter, blood pressure rises toward prepregnancy values. During the second trimester blood pressures of less than 90/60 mm Hg are frequently encountered and should be considered normal in an asymptomatic woman.
1. Clapp JF 3d, Seaward BL, Sleamaker RH, Hiser J. Maternal physiologic adaptations to early human pregnancy. Am J Obstet Gynecol. 1988 Dec;159(6):1456-60.
PMID: 3207124; UI: 89086516
2. Mooney P, Dalton KJ, Swindells HE, Rushant S, Cartwright W, Juett D Blood pressure measured telemetrically from home throughout pregnancy. Am J Obstet Gynecol 1990 Jul;163(1 Pt 1):30-6
PMID: 2197872, UI: 90328238
3. Wilson M, Morganti AA, Zervoudakis I, Letcher RL, Romney BM, Von Oeyon P, Papera S, Sealey JE, Laragh JH. Blood pressure, the renin-aldosterone system and sex steroids throughout normal pregnancy. Am J Med. 1980 Jan;68(1):97-104.
PMID: 7350810; UI: 80085432