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The fetus enters the pelvis in a cephalic presentation approximately 95 percent to 96 percent of the time. In these cephalic presentations, the occiput may be in the persistent transverse or posterior positions. In about 3 percent to 4 percent of pregnancies, there is a breech-presenting fetus (see Chapter 25). In the remaining 1 percent, the fetus may be either in a transverse or oblique lie (see Chapter 26), or the head may be extended with the face or brow presenting.

Predisposing Factors

Maternal and Uterine Factors

  1. Contracted pelvis: This is the most common and important factor

  2. Pendulous maternal abdomen: If the uterus and fetus are allowed to fall forward, there may be difficulty in engagement

  3. Neoplasms: Uterine fibromyomas or ovarian cysts can block the entry to the pelvis

  4. Uterine anomalies: In a bicornuate uterus, the nonpregnant horn may obstruct labor in the pregnant one

  5. Abnormalities of placental size or location: Conditions such as placenta previa are associated with unfavorable positions of the fetus

  6. High parity

Fetal Factors

  1. Large baby

  2. Errors in fetal polarity, such as breech presentation and transverse lie

  3. Abnormal internal rotation: The occiput rotates posteriorly or fails to rotate at all

  4. Fetal attitude: Extension in place of normal flexion

  5. Multiple pregnancy

  6. Fetal anomalies, including hydrocephaly and anencephaly

  7. Polyhydramnios: An excessive amount of amniotic fluid allows the baby freedom of activity, and he or she may assume abnormal positions

  8. Prematurity

Placenta and Membranes

  1. Placenta previa

  2. Cornual implantation

  3. Premature rupture of membranes

Effects of Malpresentations

Effects on Labor

The less symmetrical adaptation of the presenting part to the cervix and to the pelvis plays a part in reducing the efficiency of labor.

  1. The incidence of fetopelvic disproportion is higher

  2. Inefficient uterine action is common. The contractions tend to be weak and irregular

  3. Prolonged labor is seen frequently

  4. Pathologic retraction rings can develop, and rupture of the lower uterine segment may be the end result

  5. The cervix often dilates slowly and incompletely

  6. The presenting part stays high

  7. Premature rupture of the membranes occurs often

  8. The need for operative delivery is increased

Effects on the Mother

  1. Because greater uterine and intraabdominal muscular effort is required and because labor is often prolonged, maternal exhaustion is common

  2. There is more stretching of the perineum and soft parts, and there are more lacerations

  3. Bleeding is more profuse, originating from:

    1. Tears of the uterus, cervix, and vagina

    2. Uterine atony from prolonged labor

  4. There is a greater incidence of infection. This is caused by:

    1. Early rupture of the membranes

    2. Excessive blood loss

    3. Tissue damage

    4. Frequent rectal and vaginal examinations

    5. Prolonged labor

  5. The patient's discomfort seems out of proportion to the strength of ...

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