By now, your baby may be active for up to five minutes at a time. In the next few weeks, you may begin to feel some slight fluttering movements, particularly if this is not your first pregnancy (see You are 17 Weeks and 1 Day). You'll only be aware of those movements that cause your baby to make contact with the inner muscular wall of your uterus.
The placenta itself can act like a cushion absorbing the impact of all but the strongest of the baby's movements. For this reason, women with an anterior placenta (one lying on the front wall of the uterus-that closest to the skin) often feel the movements at a much later stage than those with a posteriorly sited placenta (one that lies closer to the back).
Your baby's brain is continuing to develop. The nerve cells that will form the outer gray matter start centrally within the brain, and need to move outward to their final position. This process takes place in waves that occur from 8 to 16 weeks. The migration process is not complete until 25 weeks and electrical activity cannot be detected until 29 weeks. Even after this point, gray matter continues to mature and organize neural connections in the brain throughout the pregnancy. Your baby's body is now longer than her head for the first time.