Pregnancy-Induced Hypertension (PIH)

Hypertension can exist in a pregnant woman in either of the forms: Preeclampsia-eclampsia hypertension in which after ~week 20 of the pregnancy there is protein in the urine and swelling, chronic hypertension in which protein is in the urine before week 20 or before pregnancy, and also late lypertension that is also after week 20 but does not progress preeclampsia-eclampsia does.
Preeclampsia and elampsia are dangerous because they cause blood vessels to leak. This contributes to significant weight gain and swelling in the face and upper limbs, as well as feet and hand swelling seen in many pregnancies.
If blook vessels leak in brain, significant damage can occur, such as seizures or a coma. If leaky vessels are in the liver, RBC's are abnormally destroyed, liver enzyme concentration increases, and blood clotting cell concentration decreases.
In Preeclampsia, the leaky vessels allow protein to enter the urine, often resulting in low weight of the baby.
Blood pressure increases because heart starts to work extrememly hard to make up for this leak, but capillaries are restricted from expandsion so they can't dilate to decrease work load on the heart. "an increase of 30 mm Hg in the systolic pressure, or 15 mm Hg in the diastolic pressure, or a blood pressure above 140/90 mm Hg is considered indicative of preeclampsia.
Treatment can range from bed rest or if progressing toward eclampsia, the mother can be given medications to go into labor. Eclampsia is fatal if not treated.
High blood pressure is the leading cause of death for pregnant women.

http://www.faqs.org/health/topics/34/Pregnancy-induced-hypertension.html