You will probably be very concerned until the extra-blood effect is explained, so take heart. You don't want to find yourself in a panic, figuring eight pounds every three weeks adding about sixty pounds before it's all over. Of course, if you gain six pounds every visit, you can only milk this "extra blood" explanation once.
Blood pressure actually falls a little bit during this time. But there's a self-indulgent menace approaching - the expanding uterus.
As the uterus increases in size, the less its ligaments support it. The ligaments ultimately fail altogether when the uterus is too big to fit in the pelvis. It lifts into the abdomen as an abdominal organ by the beginning of the second trimester. The Inferior Vena Cava, the main vein, which drains the entire body below, the heart, splits above the pelvis, blood flow from both legs merging into the larger blood vessel. This is the same situation with blood from the heart in the aorta, splitting into smaller arteries for each leg. But arteries, and the aorta is the largest, have their own pulsatile force and can stand up fairly well to a weight sitting on top. The flow continues. Unfortunately, veins are weaker - they don't have the tough muscular shell that arteries do and they don't pulsate, blasting their load onward. Blood flow through them is more of a negative pressure system flow, drawn up by the vacuum-like process created by the heart pumping it back out, further up.
Because of the difference in the way arteries and veins are constructed, veins are collapsible. The Vena Cava is anchored under the floor of the abdomen, so it can't move out of the way when an enlarged uterus sits right on top of it, right through the abdominal floor. This decreases the flow back to the heart, which decreases the amount of blood pumped back out of the heart through the aorta, and then your blood pressure can fall.