Just like all other human beings, babies in utero are all different from one another. What might be a totally healthy and normal level of activity for one unborn baby might be totally alarming for another. Some mothers can't feel their baby move as often or as well as others because they have an anterior placenta, which is attached at the front of the uterus. The key is to be aware of the approximate amount of your baby's movements, and to recognize when changes take place. Unborn babies sleep in the uterus and have period of waking, and period of sleeping. It is normal to expect phases with lots of movement, and times when your baby has calmed down and is not doing much. As your pregnancy goes on, you might well be able to get a feel for your baby's wake-sleep cycles and the same timing might continue when your baby reaches earth side! There is, therefore, no reason at all to worry if your baby does not kick all the time.
If you suddenly realize that you have not felt your baby move in a while, and you are worried, you can try drinking a really cold drink or gently prodding your belly to see if you will get a response. If your baby does not respond, and you feel that something is not right, getting in touch with your doctor or midwife is crucial and could even save your baby's life. Please listen to your intuition, and take action as soon as you have realized that your baby is not moving, or responding to your provocations.