Recall that Cardiac Output = Stroke Volume * Heart Rate.
Pregnant woman will have increase in both parameters but Increase in Stroke Volume is proportionally more than Heart Rate & Best answer if presented both choices in exam. (medQuest 100)
Pregnant woman will also have an increase in Plasma Volume, but this variable does not "DIRECTLY" cause increase in Stroke Volume. Increase in Plasma Volume will Increase Stroke Volume & the later one will directly increase Cardiac Output.
Blood viscosity will Decrease in pregnancy secondary to Decrease in blood hematocrit (medQuest 102). Blood viscosity is not directly involved in increase of cardiac output. Low Blood Viscosity and Plasma Volume will "INDIRECTLY" lead to increase in cardiac output (medQuest 101) due to increase in flow and thus increased Venous Return/Preload.
Hematocrit Value in Pregnancy:
Normal: 35 - 44
1st TM: 31 - 41
2nd TM: 30 - 39
3rd TM: 28 - 40
In Pregnancy, Hormone Binding Globulins will increase due to effect of Estrogen, thus Total Hormonal Level will go high (eg. Thyroid Hormone), However Free Hormone Level will be Normal. Note that, only Free Hormones play role in Feedback Inhibition/Activation.