A. Preventing preterm labor is a top reason, says Robert Atlas, MD, chief of ob-gyn at Mercy Medical Center, in Baltimore. Other conditions that may lead to a bed-rest prescription: preeclampsia; cervical changes, such as incompetent cervix or effacement (when the cervix starts to thin and stretch prematurely); a history of miscarriage, stillbirth, or premature birth; gestational diabetes; and placental complications. Moms carrying multiples also often spend time in bed. What kind of bed rest you'll need, for how long, and with what restrictions largely depends on your doctor's assessment and biases; there are no official guidelines.
The principle behind restricted activity is that lying down takes pressure off the cervix (if you are having preterm labor or have cervical issues) and allows increased nutrients to flow to the baby. Often, says Dr. Atlas, it's a form of backward logic: "Some studies suggest that women who work on their feet for 12 hours a day may be at a higher risk of preterm birth, so we make the leap that the opposite is true -- that if you stay off your feet when the pregnancy seems to be at risk, you'll keep it longer."