What is the best way to prevent movement of my baby at night?

Depends on the age: Newborns typically like being bundled. Wrapping burrito-style in a thin blanket (or purchasing a sleep sack that has a velcro closure) helps keep them confined and comfortable. As babies start to roll, they enjoy moving around. Take the bumpers and any loose bedding or other potential airway obstructions out of the crib!

Babies need movement: Newbors are prewired with random movements that help maintain muscle tone until they develop control of their body. These will occur as baby drifts thru various sleep levels. Bundling may comfort the new baby(as a return to the womb) but should give way to sleeping free. When baby can roll over, the need for back sleeping has faded. Sand bags and sleep positioners have little value.

Swaddle, not tightly: Normal, healthy new babies are happier when they are swaddled with a thin blanket. Be sure not to wrap a baby too tightly nor too warmly. Swaddling makes the baby feel snuggled and cozy, probably mimicking the small space inside the mom's uterus. After some weeks, babies do fine without swaddling. They enjoy the freedom to move their arms and legs.