Functional Anatomy of Birth

Bump To Birth

Natural Childbirth Options

Get Instant Access

The pelvic cavity is bounded above by the plane of the brim (the inner sacral promontory to the upper and inner borders of the symphysis pubis), below by the plane of the outlet (the lower and inner borders of the symphysis pubis to the end of the sacrum or coccyx), posteriorly by

Deflexion

Extension

Flexion

Deflexion

Extension

Figure 23-2 Types of fetal positions.

the sacrum, laterally by the sacrosciatic ligaments and ischial bones, and anteriorly by the pubic rami.

The birth canal, through which the infant is delivered, may be thought of as a cylindric passage with walls composed partially of hard parts (the bony pelvis) and partially of soft parts (the muscles of the pelvic floor and the pelvic ligaments). The cross section of the cylinder is oval, rather than circular, to accommodate the oval cross section of the entering fetal part (e.g., the head) as it descends through the pelvis as a result of the expulsive effect of uterine contractions.

This mechanism for delivery and its corresponding anatomy would be easily understood were it not for the fact that the long axis of the schematic oval, which lies transversely at the entrance to the pelvis, comes to lie in the anteroposterior axis in the midpelvis. The entering fetal part must therefore descend in a spiral path as it progresses through the birth canal.

The process of birth varies greatly, depending on the relationships—lie, presentation, attitude, and position—of the fetus to the maternal anatomy. It is important to define these relationships accurately to understand the birth process.

The term lie refers to the relation of the long axis of the fetus to that of the mother. In more than 99% of full-term pregnancies, the lie is in the same plane as or parallel to the long axis of the mother;this is called a longitudinal lie. In rare instances, the long axis of the fetus is perpendicular to the maternal pelvis;this is called a transverse lie.

The term presentation refers to the part of the fetus in the lower pole of the uterus overlying the pelvic brim (e.g., cephalic, vertex, breech) that can be felt through the cervix. Usually the fetus's head is flexed so that the chin is in contact with the chest. In this case, the occipital fontanelle is the presenting part, and the presentation is referred to as a vertex presentation. The fetus is in the occiput or vertex presentation in approximately 95% of all labors.

The attitude, or habitus, of the fetus is the posture of the fetus: flexion, deflexion, or extension. Figure 23-2 illustrates these postures. In most cases, the fetus becomes bent over so that the back is convex, the head is sharply flexed on the chest, the thighs are flexed over the abdomen, and the legs are bent at the knees. This is the description of the fetal attitude of flexion.

The position is the relationship of an arbitrarily chosen portion of the presenting part of the fetus to the maternal pelvis. For example, in a vertex presentation, the chosen portion is the fetal occiput; in a breech presentation, it is the sacrum; and in a face presentation, it is the chin, termed the mentum. The maternal pelvis is divided into eight parts for the purpose of further defining position. These divisions are shown in Figure 23-3.

Because the arbitrarily chosen portion of the presenting part may be either left or right, the portion can be described as left occiput (LO), right occiput (RO), left sacral (LS), right sacral (RS), left mental (LM), or right mental (RM). This part is also directed anteriorly (A), posteriorly (P), or transversely (T). For each of the three presentations (vertex, breech, and face), there are therefore six varieties of position. For example, in a vertex presentation, if the

Figure 23-3 Divisions of maternal pelvis as seen from above. Ant., anterior; L., left; post., posterior; R., right.

Figure 23-3 Divisions of maternal pelvis as seen from above. Ant., anterior; L., left; post., posterior; R., right.

Right Occiput Posterior

Was this article helpful?

0 0
Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook


Post a comment