Probably triggered by hormonal shifts and changes in blood circulation that cause the pressure in your blood vessels to decrease, headaches and dizziness occur most often during the first trimester. These symptoms may also be brought on by fatigue, sinus congestion, stress, hunger, and/or caffeine withdrawal. Though usually nothing to be concerned about, you should tell your healthcare provider if you're experiencing dizziness. Very early in pregnancy, dizziness is a symptom of ectopic pregnancy, especially when severe and accompanied by abdominal pain or bleeding. Painful migraine-type headaches and/or fainting should also be reported to your practitioner.