bubble_chart Overview

Labor forces include uterine contractions, abdominal wall and diaphragmatic muscle contractions, and levator ani muscle contractions, with uterine contractions being the primary component. During childbirth, abnormal uterine contractions are characterized by irregularities in rhythm, symmetry, polarity, intensity, or frequency. Clinically, this often results from obstructive dystocia caused by abnormalities in the birth canal or fetal factors, which increase resistance during fetal passage through the birth canal, leading to secondary abnormalities in labor forces. Abnormal uterine contractions are clinically classified into two categories: uterine inertia and hypertonic uterine contractions. Each category is further divided into coordinated and uncoordinated uterine contractions.

bubble_chart Etiology

It is often caused by a combination of several factors, with common reasons including:

1. Cephalopelvic disproportion or abnormal fetal position: The descent of the fetal presenting part is obstructed, preventing it from closely adhering to the lower uterine segment and cervix, thereby failing to trigger reflexive uterine contractions, leading to secondary uterine contraction lack of strength.
2. Uterine factors: Uterine hypoplasia, uterine malformations (such as bicornuate uterus, etc.), overdistension of the uterine wall (as in twins, macrosomia, polyhydramnios, etc.), multiparous women with degenerative changes in uterine muscle fibers or uterine fibroids, all of which can cause uterine contraction lack of strength.
3. Psychological factors: Primiparous women (especially elderly primiparas over 35 years old) who experience excessive mental stress, leading to dysfunction of the cerebral cortex, insufficient sleep, reduced food intake during labor, and excessive physical exertion, can all result in uterine contraction lack of strength.
4. Endocrine disorders: After labor, insufficient secretion of estrogen, oxytocin, prostaglandins, acetylcholine, and slow decline in progesterone levels, as well as reduced uterine sensitivity to acetylcholine, can all affect the excitation threshold of uterine muscles, causing uterine contraction lack of strength.
5. Drug effects: Inappropriate use of large doses of sedatives and analgesics during labor, such as morphine, chlorpromazine, pethidine, barbiturates, etc., can inhibit uterine contractions.

bubble_chart Clinical Manifestations

According to the time of occurrence, it can be divided into primary and secondary types. Primary uterine contraction lack of strength refers to the onset of labor where uterine contractions are weak from the beginning, the cervix fails to dilate as expected, the fetal presenting part does not descend as expected, and labor is prolonged. Secondary uterine contraction lack of strength refers to normal uterine contractions at the onset of labor, but as labor progresses to a certain stage (often during the active phase or the second stage of labor), uterine contractions weaken, labor progress slows, or even stalls. Uterine contraction lack of strength has two types, with different clinical manifestations.

1. Coordinated uterine contraction lack of strength (hypotonic uterine contraction lack of strength): Uterine contractions have normal rhythm, symmetry, and polarity, but the contraction force is weak, intrauterine pressure is low (<2.0 kPa), duration is short, the interval is long and irregular, and contractions occur less than twice per 10 minutes. When uterine contractions reach their peak, the uterine body does not bulge or harden, and pressing the uterine fundus with fingers may still leave an indentation. Labor is prolonged or stalls. Due to low intrauterine tension, the impact on the fetus is minimal.
2. Uncoordinated uterine contraction lack of strength (hypertonic uterine contraction lack of strength): The polarity of uterine contractions is reversed. Contractions do not originate from the two cornua of the uterus; instead, the excitation points come from one or multiple areas of the uterus, resulting in uncoordinated rhythm. During contractions, the uterine fundus is not strong, but the middle or lower segment is. During the interval, the uterine wall does not fully relax, manifesting as uncoordinated uterine contractions. These contractions cannot dilate the cervix or descend the fetal presenting part, making them ineffective. The mother experiences persistent lower abdominal pain, tenderness, dysphoria, restlessness, dehydration, electrolyte imbalance, intestinal distension, and urinary retention. Fetal-placental circulation is impaired, potentially leading to fetal distress. Examination reveals tenderness in the lower abdomen, unclear fetal position, irregular fetal heart rate, slow or absent cervical dilation, delayed or stalled descent of the fetal presenting part, and prolonged labor.

Abnormal labor curves

Uterine contraction lack of strength leads to abnormal labor curves, which can manifest in the following seven types:

1. Prolonged latent phase: The latent phase is defined as the period from the onset of regular labor contractions to cervical dilation of 3 cm. For primiparas, the normal latent phase lasts about 8 hours, with a maximum limit of 16 hours. Exceeding 16 hours is termed a prolonged latent phase.
2. Prolonged active phase: The active phase is defined as the period from cervical dilation of 3 cm to full dilation. For primiparas, the normal active phase lasts about 4 hours, with a maximum limit of 8 hours. Exceeding 8 hours is termed a prolonged active phase.
3. Active phase arrest: After entering the active phase, if cervical dilation ceases for more than 2 hours, it is termed active phase arrest.
4. Prolonged second stage: For primiparas, if the second stage exceeds 2 hours, or for multiparas, if it exceeds 1 hour without delivery, it is termed a prolonged second stage.
5. Second stage arrest: If there is no progress in fetal head descent for 1 hour during the second stage, it is termed second stage arrest.
6. Prolonged descent: During the active advanced stage, when cervical dilation reaches 9–10 cm, if the fetal head descends at a rate of less than 1 cm per hour, it is termed prolonged descent.
7. Descent arrest: If the fetal head remains stationary for more than 1 hour without descending, it is termed descent arrest.

These seven types of abnormal labor progress can occur individually or in combination. When the total duration of labor exceeds 24 hours, it is termed prolonged labor, which must be avoided.

bubble_chart Treatment Measures

Whether it is primary or secondary, once there is a lack of strength in coordinated uterine contractions, the first step is to identify the cause, such as cephalopelvic disproportion or abnormal fetal position, and to assess cervical dilation and fetal descent. If cephalopelvic disproportion is detected and vaginal childbirth is deemed impossible, a timely cesarean section should be performed. If no cephalopelvic disproportion or abnormal fetal position is found and vaginal childbirth is considered feasible, measures to strengthen uterine contractions should be considered.

First Stage of Labor

1. General Management: Relieve mental tension, encourage rest, and promote adequate nutrition. For those unable to eat, intravenous nutrition should be provided, such as 500–1000 ml of 10% glucose solution with 2g of vitamin C. In cases of acidosis, 5% sodium bicarbonate should be supplemented. For hypokalemia, potassium chloride should be administered via slow intravenous drip. For overly fatigued parturients, 10mg of diazepam can be given via slow intravenous injection or 100mg of pethidine via intramuscular injection. After some time, uterine contractions may strengthen. For primiparas with cervical dilation less than 3 cm and intact membranes, a warm soapy water enema can be administered to stimulate intestinal peristalsis, expel feces and gas, and promote uterine contractions. If spontaneous urination is difficult, induction methods should be attempted first; if ineffective, catheterization should be performed, as emptying the bladder can widen the birth canal and enhance uterine contractions.
2. Strengthening Uterine Contractions: If uterine contractions remain weak after general management and a diagnosis of coordinated uterine contraction lack of strength is confirmed with no significant progress in labor, the following methods can be used to strengthen contractions: Artificial Rupture of Membranes: If cervical dilation is 3 cm or more, there is no cephalopelvic disproportion, and the fetal head is engaged, artificial rupture of membranes can be performed. After rupture, the fetal head directly presses against the lower uterine segment and cervix, triggering reflexive uterine contractions and accelerating labor progress. Some scholars advocate for artificial rupture of membranes even if the fetal head is not engaged, believing it can promote fetal descent into the pelvis. Before rupture, it is essential to check for umbilical cord prolapse. Rupture should be performed during a contraction interval, and the operator’s fingers should remain in the vagina for 1–2 contractions until the fetal head descends before removal. Bishop proposed a cervical maturity scoring system to predict the effectiveness of measures to strengthen contractions (see Table 1). If the score is 3 or below, artificial rupture of membranes is likely to fail, and alternative methods should be used. Scores of 4–6 have a success rate of about 50%, 7–9 about 80%, and scores above 9 are almost always successful.

bubble_chart Prevention

Pregnant women should receive prenatal education to alleviate their concerns and fears, helping them understand that pregnancy and childbirth are physiological processes. Currently, maternity wards both domestically and internationally offer comfortable labor rooms (where partners and family members can accompany them) and family-style wards, which help reduce the mother's tension, boost confidence, and prevent uterine inertia caused by mental stress. During childbirth, mothers are encouraged to eat more, and intravenous nutrition can be supplemented if necessary. Avoiding excessive use of sedatives and checking for conditions like cephalopelvic disproportion are effective measures to prevent uterine inertia. It is also important to ensure timely emptying of the rectum and bladder, with warm soapy water enemas or catheterization performed if needed.