Dizzy Spells in Pregnancy
There are many things that can cause dizzy spells during pregnancy. Even though most people believe it's due to the anemia of pregnancy, this is actually a rare cause. The most likely causes are fluid shifts that can affect heart rate, which wouldn't necessarily do anything unless you stood up suddenly and your blood were to pool in your legs. Up to 40% of your entire blood supply can pool in your legs, decreasing blood supply to your heart, affecting blood pressure, heart rate, and ultimately blood flow to your brain, giving your 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, you should report dizziness to your doctor anyway. Anemia in pregnancy would have to be extremely severe to cause dizziness, but take your iron anyway. Additionally, the anemia in pregnancy is a gradual decline, so that something you're already used to shouldn't be a cause for dizziness.
Avoid Pregnancy Dizziness by:
Examining your eating habits--stay away from sudden carbohydrate loads
Keeping well hydrated
Avoiding standing for long periods at a time with the knees locked or standing up suddenly after sitting for a time