It is quite normal to feel a little dizzy and lightheaded while you are pregnant. Most women experience these symptoms due to the normal body changes occurring during this time . However, in some rare cases they may be associated with serious complications.
It is often considered a symptom of pregnancy occurring early in the first trimester , sometimes even before one notices a missed period, along with other signs like fatigue, nausea and diarrhea. However in many women, dizziness does not occur until the second or even third trimesters .
The cardiovascular changes are principally responsible for the feeling of lightheadedness  as pregnancy increases the blood volume in your body (by around 50%) to accommodate the needs of the fetus. Additionally, the higher levels of pregnancy hormones cause your blood vessels to dilate, lowering your blood pressure which reaches its lowest point at around mid or late second trimester. The low BP levels often hamper the blood flow to the brain, leaving you dizzy or faint . It may also result from high blood pressure levels towards the last trimester. Expecting twins, having a history of low blood sugar, anemia or diabetes and suffering from varicose vein or gestational diabetes may increase your risks of developing this symptom . The growing uterus, putting pressure on your blood vessels, may lead to dizziness. Additionally, lying on your back in the third trimester makes the baby press on the vena cava (the principal vein carrying blood to the heart from the lower body), making you feel lightheaded.
It is not always possible to prevent the lightheadedness associated with pregnancy. But, it can be easily managed with certain measures:
Persistent or recurring dizziness accompanied by one or more of the following symptoms may indicate complications like placental abruption, ectopic pregnancy , pre-term labor and contractions  or miscarriage :
The ICD-9 code used for dizziness is 780.4  while its ICD-10 code is R42