I don't have a good section to quote in particular, but Chesley's Hypertensive Disorders in Pregnancy, Second Edition talks about a decrease in pressures around 16-20 weeks. It is a bunch of medical jargon, but I'll give you a quote. It has some great graphs, but I don't have a scanner and don't even know if that's legal??? lol The graphs start at about 14 wks gestation and show a decrease from there with the lowest point being around 16-24 weeks with blood pressures increasing gradually toward the end of pregnancy and passing the 14 week pressures. The ammount of change is small in my opinion (averaging a decrease of less than 10 by the graph) and so may not be easily noticed unless you are keeping close track of your Blood pressures.
Here's a quote: "In general, mean arterial pressure declines in the first trimester, reaching a nadir between 16-20 weeks' gestation with a return to control values near term. Diastolic pressures decline to a greater extent than systolic pressures, the average mean change being about 10 mm Hg." (Lindheimer, Roberts, Cunningham, pg. 70)
Sorry, trying my best to pretend I have some knowledge of how to site a work, but I honestly don't remember! lol
DH Britton (27)
Emma Margaret (03/02/03) 37 weeks from PIH & oligo