Toward the end of the middle trimester and throughout the last one many women are awakened by knot-like cramps in their legs. These cramps are sometimes blamed on an electrolyte imbalance of calcium, phosphorus, magnesium, and potassium. An additional explanation
is the decreased circulation to the most active muscles in your legs. Pressure of the uterus on major blood vessels, as well as standing, sitting, or lying for a long time, can slow blood supply to these muscles, causing them to cramp up.