These belong to a group of drugs called narcotics (which literally means sleep-inducing), which includes morphine. They attach themselves to receptors in the brain or nerves and block the transmission of pain.
These drugs don't relieve pain, but they can help to relieve anxiety and help a women relax for several hours during early labor. Some women who are very anxious about the impending pain or of labor and childbirth may seek this sort of medication. Tranquilizers may be given orally, intravenously, or injected into a large muscle. Oral dosage takes the longest to take effect. Some women dislike what tranquilizers do to them, since they may feel drowsy and out of control. If the dose is very high, women may nod off between contractions. It may be hard for a woman to fully remember her labor experience if she's taken tranquilizers. The drugs also effect the baby, decreasing his activity and muscle tone during and after birth. For these reasons, tranquilizers are not for everyone, but in certain circumstances, they can help a woman feel less anxious about labor.