Hormones are raging through your body in enormous quantities, and your body is going through more changes than ever before. Your blood vessels are widening, and some of your blood flow is being diverted to your uterus. This can lead to low blood pressure (hypotension), and make you feel dizzy. Additionally, if you are suffering from heavy morning sickness and vomit a lot, dehydration can contribute to feelings of dizziness. Making sure that you stay hydrated and drink enough water is of key importance during this time. Some women are hospitalized for dehydration in pregnancy. The good news is that the vast majority of pregnant women can simply avoid dehydration by drinking plenty of water.
Another possible cause of dizziness in the first trimester is the fact that blood sugar levels drop during this time of your pregnancy. This is especially true for those with low iron levels, or anemia. There are some steps you can undertake to gain relief from dizziness during your first trimester. Eating regular, small meals keeps your blood sugar levels constant. Drinking enough water is so important that I will mention it yet again. And not standing or for long period of time helps a lot too. Avoiding overheating is another thing that helps, but is sometimes difficult to achieve. You can't help the weather outside, but you can make sure you don't have hot baths or showers.