A muscle cramp during pregnancy may occur in any muscle or muscle group, but is most commonly experienced in the legs. Muscle spasms that occur in the back or abdomen are also possible. A muscle cramp or spasm is characterized by the following: 1  Involuntary contraction or tightening of a muscle. 2  Usually sudden.
During breathing, these muscles normally tighten and pull the rib cage up. Your chest expands and the lungs fill with air. Intercostal retractions are due to reduced air pressure inside your chest. This can happen if the upper airway (trachea) or small airways of the lungs (bronchioles) become partially blocked.
There are three principal layers; 1  External intercostal muscles aid in quiet and forced inhalation. 2  Internal intercostal muscles aid in forced expiration (quiet expiration is a passive process). 3  Innermost intercostal muscle, the deep layers of the internal intercostal muscles which are separated from them by a neurovascular bundle.