Many pregnant women complain of dizziness and lightheadedness throughout their pregnancies. Fainting, a feeling of dizziness or vertigo that comes on suddenly, is rare but does happen even in some healthy pregnant women. Dizziness or fainting will make you unsteady on your feet. This occurs if you stand up too quickly or if you stand for too long. You may be especially prone to it in hot weather.
There are many reasons for these symptoms. The growth of more blood vessels in early pregnancy, the pressure of the expanding uterus on blood vessels, and the body's increased need for food all can make a pregnant woman feel lightheaded and dizzy. Pooling of the blood in the legs and feet when standing can lead to a lack of blood supply to the brain, causing dizziness or fainting. The uterus also demands an increased blood supply, leading to less blood going to the brain.
Call your doctor if you feel faint and have vaginal bleeding or abdominal pain.
There's generally no risk to your baby unless you fall on your stomach.