The second stage, from the complete dilatation of the os to the delivery of the child.
The third stage, from the delivery of the child to the expulsion of the placenta.
The first stage is the stage of dilatation.
Usually at term, the cervix is columnar and unshortened, the canal intact, and closed at both ends, as shown in Fig. 36.
In multiparas the outer opening will usually admit the tip of the finger.
As labor proceeds, the cervix is effaced, the os slowly dilates, and the bag of waters forms.
The Bag of Waters.—When the cervix is effaced and only the os remains, the lower end of the egg with its fluid restrained by the membranes, bulges forward into the canal. The fœtal head, or breech presses into the pelvis, and the fluid in the membranes, compressed between the presenting part above and the cervix below, is called the bag of waters.
When the contraction comes on the longitudinal muscular fibers of the uterus are drawn upward and the bag of waters becomes tense and pushes farther and farther down into the opening; and by its even and universal pressure, mechanically and slowly increases the size of the opening which the muscular traction is pulling apart. At the same time, the fluid around the child prevents, for a time, direct and injurious compression on the body. When no definite cervical projection can be felt, and when the teat-like protrusion of the cervix has disappeared, the cervix is said to be effaced.
Fig. 35.—The bag of waters begins to act on the cervix. (Eden.)