Early in your first trimester, you may feel dizzy and go through bouts
of nausea as your blood flow works to catch up with your rapidly
expanding circulatory system. In the second trimester, your growing
uterus can put pressure on blood vessels, which can also make you feel
dizzy. And it's not uncommon to feel dizzy occasionally throughout
pregnancy for a number of other reasons: You might feel dizzy if you
haven't eaten in a while and your blood sugar is low, especially in
the late afternoon, if you're overheated, or if you stand too quickly
(also called postural hypotension).
The best first step? Sit down. Not only will that help the dizziness
subside, but also you'll keep yourself from falling. If you're in a
stuffy office or crowded subway, then fresh air may be a solution. Or
if you haven't eaten for an hour or two, a quick, healthy snack will
help boost your blood sugar. Also try drinking some water or juice.
(In general, keep yourself well hydrated by drinking at least eight to
ten glasses of water a day.) Simply moving into a lower gear can help,
for instance, getting up slowly and smoothly instead of springing
from a chair or bed. In some cases you can't do anything except sit or
lie down and wait it out. Waves of dizziness can and probably will
crash down upon you at any time during pregnancy.
National Geographic - In the Womb
Paralumun New Age Village