Your Uterus contracts all the time (in the same way that your heart does) but you will be unaware of this when you are not pregnant. When you are pregnant however, you may become aware of these tightenings or contractions (known as Braxton Hicks) from around 16 weeks. Most women may not notice them until they are towards the end of their second or third trimester when the uterus is much larger. Some women will not notice them at all. For most women these are totally painless and have no clinical importance. But as the pregnancy progresses they can become uncomfortable and even take your breath away.
Braxton Hicks contractions are random and there is no rhythmic pattern to them- they are not associated with show of blood or any other worrying signs. Many women do not start to feel them until late stages in their pregnancy or even in the few days before labour begins, and some claim never to experience them at all. Many women experience them without even feeling them and are thus unaware that they ever had them. These tightening feelings can be quite surprising or alarming when they first start but they are perfectly normal and many women experience in abundance them right up until birth. They do not indicate anything serious, nor do they mean you will go into labour earlier than someone who does not feel them. You may become more aware of these in successive pregnancies.