While doctors aren't sure what triggers the onset of labor, there are numerous way to tell that a baby is about to be born, such as when a pregnant women feels a heaviness on her pelvis, a reduction of pressure underneath the ribcage and frequent contractions, according to BabyCenter. The contractions become very intense as well, which also signal other changes the body is going through to prepare for labor.

As the contractions become more frequent and intense, the cervix starts to ripen, preparing itself for the passage of the child. In this process, the cervix softens and begins to dilate a centimeter or two. Some women report having a cramp-like feeling in the stomach, a common pain during menstruation. When child birth gets very near, contractions, also referred to as Braxton Hicks contractions, become very sudden, sharp and very painful. They happen often, around every 20 minutes.

Child birth begins to happen when the contractions grow longer and start to connect together into one long contraction. When this doesn't happen, however, doctors refer to this as false labor.

In the days before labor, women also commonly experience a large amount of vaginal bleeding. This occurs because their changing cervix passes the mucus plug, which is the mucus that sealed the cervical canal during the last nine months of pregnancy.

When a woman's water breaks, this is usually a sign that child birth is imminent. The "water" refers to an amniotic sac of fluid that surrounds and protects the baby. When the baby is ready to be born, this sac becomes obsolete, ruptures and excretes out the body through the vagina.