A slight increase at any given time is usually nothing to worry about. It could have been caused by stress, nervousness, anxiety, or illness.
If your blood pressure remains slightly elevated, you may be among the 1-2% of pregnant women who develop transient high blood pressure during pregnancy. This type of hypertension is harmless and disappears after delivery.
What is considered as normal blood pressure readings in pregnancy varies a little over the course of the nine months. Your normal blood pressure generally drops a big over the first several months and this begins to rise a little by the 7th month.
During the first or second trimester, if systolic pressure (the top number) is over 130 or rises by 30 over the your normal readings or diastolic pressure (the bottom number) is over 85 or rises by 15 over your normal readings, and stays up for at least two readings taken at least six hours apart, close observation and possibly treatment is warranted. In the third trimester, treatment is usually only used if the rise is greater than that.
If such an increase in blood pressure is accompanied with sudden weight gain (more than 2 pounds that isn't related to overeating), severe edema, particularly in the hands, face, and ankles, and/or protein in the urine, the problem may turn out to be preeclampsia (aka pregnancy induced hypertension or PIH). Edema is swelling due to water retention.
In women who receive regular medical care, preeclampsia is diagnosed before it progresses to more serious symptoms. These symptoms include blurred vision, headaches, irritability, and gastric pain.
If you experience any symptoms of preeclampsia, you should call your doctor immediately.