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Summary: Forearm or calf venous plethysmography has been used in pregnant women to examine the effects of pregnancy on the vascular system and more recently to measure blood flow changes in response to intra-arterial infusion of vasoactive substances. To examine the assumption that venous capacitance and blood flow measurements are the same in the forearm and calf, venous plethysmography was conducted simultaneously on the forearm and calf in normal (NP) and hypertensive pregnant women (HTP) in their third trimester and in normal nonpregnant women (N). Ail studies were made on the right forearm and calf with subjects in the left lateral recumbent position. There was no significant difference between venous capacitance in the forearm (median: 124 times 103rruV100mIVmrnHg for both NP and HTP) and leg (134 [NP] and 106 [HTP] x lO'mL/lOOmlVmmHg) for both pregnant groups but venous capacitance in the nonpregnant group was greater in the leg than arm (174 versus 112 x lO'mUlOOmL/mmHg, p <0.001). Blood flow was similar in the leg and forearm (median: 4.9 versus 3.9mL/100mL/min respectively) in nonpregnant women. Forearm blood flow was also similar to calf flow in NP women (6.2 versus 4.3mL/100mL/min respectively) but greater man calf blood flow in HTP (9.7 versus 5.3ml7100mL/min, p <0.01). Using left lateral recumbency, forearm and calf vein capacitances are similar in pregnant women, in contrast to nonpregnant women. Blood flow is higher in the forearm than the calf only in hypertensive pregnant women. These observations should be borne in mind when interpreting studies of regional blood flow in pregnancy.