My 6 month old is not sitting up yet! Should I be worried? She does everything else the baby books say she should be doing.
One of the most common concerns for a parent is whether or not her child is developing normally. The one area of development that tends to cause the most concern is gross motor development. Although many baby books provide lists of "what your baby should be doing" at various ages, few books explain just what "normal development" is and why there is a lot of room for variation.
To help parents understand development, it is important to know that the brain is fairly compartmentalized. The part of the brain that tells the shoulders to move is separate from the part the tells the right thumbs to move. The right side of the brain controls the movements on the left side of the body and vice versa. Therefore, movement of the right shoulder is controlled by a separate area of the brain from that part that controls the left shoulder. This part of the brain that controls muscle movement is called the "motor strip". The motor strip is separate from the part of the brain that interprets spoken speech, stores vocabulary words, recognizes symbols, signals the urge to breath, et cetera!
By the time a baby is born after nine months of pregnancy, the signaling from the brain is still immature. The part of the brain that controls basic functions like sucking and swallowing and breathing is pretty much ready to go, but the part that controls the neck muscles, arms, legs, etc is still under development. Furthermore, although the brain has to signal to the rest of the body, it is not the whole story when it comes to movement. Nerves receiving signals from the brain must also be in good working order. The nerves send chemicals from one to another as signals. For the signal to travel down the nerve (think electricity going down a wire), the nerve must also be properly insulated with a substance called myelin. Then the muscles that receive the signal must be able to respond. The whole process is extraordinarily complicated on a cellular level and it is a wonder the most of us do as well as we do!
The maturational process occurs fairly predictably from the top down. First head control is gained as the nerves supplying the neck muscles mature and the neck muscles gain strength. The shoulder, upper arms, and hand control improves. Upper body or trunk control is next, then the hips and pelvis, and finally the legs! Because hand movements are so complex, doctors track "fine motor" skills separately from gross motor skills. Balance and coordination are part of gross motor skills although they have more complex controlling systems than just the motor strip. Thus the familiar order of gross motor development: turning the head from side to side when lying prone (i.e., on the stomach) at two weeks, then lifting the head briefly when prone at two to three months, holding the head upright by four to five months, then raising up the shoulders and upper chest when four to five months. Arm movements also gain some control about age to four to five months as the infant can bring her hands to midline when lying on her back. After that, she can reach and grab an object and bring it to her mouth. She can hold an object in one hand and transfer it to another by six to seven months. Truncal control allows the baby to sit propped against her outstretched arms about six to seven months with independent sitting by eight to nine months. Finally, pulling up as the signals to the pelvis and thigh muscles increases followed by standing and then walking. After that, signaling continues to be refined and allows the child to run, climb, hop on two feet, then one foot, then skip by age five.
Because gross motor skills are so obvious, these are the ones that parents tend to notice the most. But remember, gross motor is only one part of development. If gross motor skills are associated with delays in the other areas of development, this is more concerning for the child’s overall well-being. If gross motor skills are the only area that is lagging, this is usually not as concerning for long term well-being. There is large variability in the age at which children attain most gross motor milestones. If you have any concerns, always talk to your child’s pediatrician or family practitioner.
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