Why does preeclampsia occur?

Preeclampsia affects about 1 in 25 pregnancies in the U.S. It usually begins after the 20th week of pregnancy and causes high blood pressure, protein in the urine, and swelling. The cause is not entirely known but recent studies suggest that it occurs when the placenta doesn’t anchor itself deeply within the wall of the uterus during the first trimester which could be influenced by the immune system, genetics, or underlying health conditions such as diabetes or high blood pressure.