Your baby's muscle tone takes some time to develop fully. By this week of your pregnancy, head control is improving and in the legs muscle tone now allows for smoother and more complex movements. For once, the arms and hands lag behind the legs and feet in development, taking another three weeks to achieve the same level of tone and movement.
In the last few weeks the familiar "fetal position" is increasingly seen on scans: this is not only due to lack of space, but also because your baby's flexor muscles (those that bend the elbows, hips, and knees) have a better tone than her extensor muscles (the muscles that extend the arms and legs) in each limb.
You will also be aware that your baby is moving more now than at any other time. You only feel the movements that hit the lining of your uterus, but there will be many more small movements of which you are completely unaware. Movements inside the uterus are important: they help your baby's coordination, strengthen her bones, and increase muscle mass. The number of muscle cells increases up until 38 weeks. From this point on, individual muscle cells lengthen and expand in response to exercise, further increasing muscle mass and strength.