Muscle cramps are common during pregnancy, especially in the later stages. The cause is unknown, but cramps often occur if you've strained the muscle more than usual or stayed in an uncomfortable position for an extended period. Cramps usually subside on their own within a few minutes, but gently stretching the muscle, massaging it, or taking a few steps can help alleviate discomfort.
To prevent cramps, regularly stretch and not just when cramps occur. Strengthening the calf muscles through toe raises can also help avoid leg cramps by making the muscles more resilient to increased strain.