There are many things that can cause dizzy spells in pregnancy. The common notion is that dizziness is a result of the anemia of pregnancy, which means that since your blood supply increases dramatically during pregnancy your red blood cell count is diluted potentially leading to anemia. But the anemia of pregnancy is a rare cause of dizziness and you should know that the anemia would have to be extremely severe to cause dizziness.
The most likely cause are fluid shifts that can affect your heart rate. For example, up to 40 percent of your entire blood supply can pool in your legs (if you’re sitting or standing in the same position for a long time), decreasing blood supply to your heart, affecting blood pressure, heart rate, and ultimately blood flow to your brain, giving you dizziness.
Another common culprit is a low blood sugar. This can happen easily in pregnancy with even one skipped meal. And as strange as it may sound, even a sudden high blood sugar can cause a sudden subsequent crash in sugar levels (by causing you to overshoot on your own insulin which brings your sugar down too much–called the “Somoygi” effect, also spelled “Somogyi”).
Cardiac arrhythmias can lead to dizziness, and although this is uncommon. No matter what the suspected cause, you should report dizziness to your health care provider anyway.