Each year nearly one in five pregnant women, develops a pregnancy complication that results in a bed rest order. In the first trimester, it’s usually because of vaginal bleeding that threatens a miscarriage, says Dr. Michele Hakakha, M.D., OB/GYN and author of Expecting 411. In the second trimester, the most common concern is cervical insufficiency -- which means the cervix opens or shortens long before the baby is ready to come out (which cam result in early labor). In the third trimester, bed rest is often prescribed to stop contractions (or in the case of carrying multiples, to prevent them from starting too soon). Bed rest works by reducing pressure on the cervix, keeping it from opening too soon, and from starting contractions.