First stage of labor.pptx physiology of first stage of labor

first stage of labor

1. Formation of Upper segment and lower segments By the end of pregnancy, the body of the uterus is divided into two anatomically distinct segments, i.e. upper and lower segments. The upper segment is mainly concerned with contraction and is thick and muscular. the lower segment is prepared for distension and dilatation, and is thinner. The lower segment develops from the isthmus and is about 8-10cm in length.

2. Uterine Action Fundal Dominance: each uterine contraction starts in the fundus near one of the cornua and spreads across and downwards. Each contraction is longest and intense in the fundus, but the peak is reached simultaneously over the whole uterus and fades from all parts together. This pattern permits the cervix to dilate and the fundus to expel the fetus.

3. Polarity A neuromuscular harmony prevails between the two poles or segments of uterus throughout labor, which is termed as polarity. During each contraction these two poles act harmoniously. The upper pole contracts stronger and retracts to expel the fetus, the lower pole contracts slightly and dilates to allow expulsion to occur. If polarity is disorganized, the progress of labor is inhibited.

4. Contraction & Retraction During labor the contraction does not pass off entirely, the muscle fibers retain some of the shortening of contraction instead of becoming completely relaxed. This is termed retraction which is a unique property of uterine muscle. Because of this the upper segment of the uterus becomes gradually shorter and thicker and its cavity diminishes assisting in the progressive expulsion of the fetus.

5. When labor begins, the uterine contractions occur every 15-20 minutes and may last for 30 seconds. They are weak to begin with and occur with rhythmic regularly and intervals between them gradually lessen. By the end of first stage they occur at 2-4 minutes interval, last for 50-60 seconds and are very powerful.

6. Retraction Ring/Constriction Ring A ridge forms between the upper and lower uterine segments, which are known as the retraction ring. The physiological retraction ring gradually rises as the upper retraction ring gradually rises as the upper uterine segment thins out to accommodate the descending fetus. Once the cervix is fully dilated and the fetus can leave the uterus, the retraction ring rises no further it is also called as Schroeder ring.

7. Cervical Effacement It is defined as thinning of cervix and shortening of the cervix canal from its usual length of 2-3cm to one in which the cervical canal is obliterated leaving only the external os as a circular orrifice with thin edges. Effacement is clinically evaluated in terms of percentages (50%, 80%).

8. Cervical Dilatation Dilatation of the cervix is the process of enlargement of the external cervical os from an orifice of a few millimeters to an opening large enough to pass the baby out. Dilatation is effected primarily by the action of contractions and facilitated by hydrostatic action of the amniotic fluid under the influence of contractions. Dilatation is clinically evaluated by measuring diameter of the cervical opening in centimeters, with 0cm being closed and 10cm complete dilatation.

9. Mechanical Factors Formation of Forewaters General Fluid Pressure Rupture of Membranes Fetal Axis Pressure Descent of Presenting part

10. Formation of Forewaters

11. General Fluid Pressure While the membranes remain intact, the pressure of the uterine contractions is exerted on the fluid and, as fluid is not compressible, the pressure is equalized throughout the uterus and over the fetal body.

12. Rupture of Membranes The physiological moment for the membranes to rupture is at the end of first stage of labor when the cervix becomes fully dilated and no longer supports the bag of waters. Occasionally the membranes do not rupture even in the second stage and appear at the vulva as a bulging sac covering the fetal head as it is born, this is known as the CAUL.

13. Fetal Axis Pressure During each contraction the uterus rears forward from upper segment to lower segment, towards the long axis of the fetus.

14. Descent of Presenting Part