26 weeks pregnant: blood pressure - 123/70. Is this within range? At what point do I need to worry about pre-eclampsia?

140/90. The blood pressure criteria for pre-eclampsia is 140/90 however a diagnosis also requires protein in the urine. Nevertheless having high blood pressure alone is also associated with complications so you make sure you continue to consult w/ your OB.
Your BP is normal. A number of signs and symptoms may be present with pre-eclampsia. Elevated BP >140/90 and the presence of protein in your urine. These are things your doctor should check for in addition other blood tests and collecting your urine for 24 hours to check total amount of protein.