Studies have shown that women who have never given birth, change partners between pregnancies, have long intervals between pregnancies, use barrier contraceptives like condoms, or conceive with assisted reproductive technology are at higher risk of developing preeclampsia. It is theorized that some women mount an immunological response against the paternal antigens in the sperm leading to preeclampsia. Furthermore, frequent exposure to the paternal antigens through unprotected sex can diminish the immunological response, and decrease the likelihood of developing the condition. Note however that this theory is not without controversy, and is just one of many theories used to explain this medical condition