It sounds like you have vasovagal syncope. This is a relatively common reaction to having your blood drawn. One study found that it occurs in 2 - 5 percent of patients who donate blood. If you do have vasovagal syncope, you should have your blood drawn when lying down. It won't necessarily prevent you from fainting, but it will help keep you from injuring yourself if you do pass out. Hydrating with up to a half-liter of water prior to your blood draw may also help prevent these episodes. In general, though, vasovagal syncope is not harmful to the baby since the episodes last only for a short time and are self-limited.