it's quite normal for blood pressure to drop early in pregancy as the blood vessels dilate and blood volume also increases to accomodate the growing baby. Blood pressure in fact reaches the lowest point somewhere in middle of second trimester and then starts to rise again and returns to normal after childbirth.
Some women may experience dizziness and lightheadedness due to this but if symptoms persist you may have to see your obstetrician as there maybe other reasons too ie anemia, dehydration, heart or endocrine issues and that the pregnancy is proceeding as expected.
Generally BP below 90/60 would be classed as hypotension though most physicians would go by symptoms and not by numbers. Many people go through life with low blood pressure numbers but absolutely no symptoms or ill effects.
While awaiting review with your obstetrician I would suggest the following.avoid dehydration, esp. on hot days/ ensure you have adequate nutrition especially if nausea or vomiting is present/smaller meals might help/avoid standing up for prolonged periods/avoid standing up or shifting posture suddenly/wear support stockings if needed/avoid lying on your back- lying on the left side is preferable/avoid severe exertion and very hot baths. Hope this helps!
