The cause of dizziness in the third trimester is probably your baby pressing down on your major blood vessels. As you baby's birth nears, your uterus has expanded so much that the sheer size of it means that other internal organs are getting squashed. Because of that, less blood is being circulated to your head. Your blood pressure might also be dropping around this time during your pregnancy, and you are likely to be low on energy. All of that means that you might be experiencing dizzy spells. Unless you have a whole host of other symptoms that cause you to worry as well, dizziness is not something you need to be concerned about, though it can certainly be very annoying. The best thing to do when you feel dizzy is to sit down or lie down, and stop what you are doing. Eating and drinking something may also be of help, because both dehydration and low blood sugar levels can contribute to your dizzy spells.
So, when do you need to call your doctor? If you are experiencing contractions, sharp abdominal pain, or headaches, or if you simply have a feeling that something is not right. On the whole, dizziness in the last few weeks of your pregnancy is no more than a signal that you need to take things easy, and prepare for the birth of your baby. Put your feet up, while you still can!