What is Hypotonic Labor?
Hypotonic labor refers to an abnormal delivery process. It mainly becomes noticeable during the active stage of labor and is marked by weak and insufficient uterine contractions that fail to adequately lead to the expansion (dilation) of the cervix, the thinning (effacement) of the cervix, and the movement of the baby downwards. This can result in a lengthy or prolonged birth process.
A typical labor is divided into three stages, and the first stage can further be broken down into two phases: the latent phase and the active phase. During the active phase, we anticipate uterine contractions to be more frequent, stronger and last longer. But in hypotonic labor, these contractions are not as expected.
The progress of a normal labor is influenced by four factors, often referred to as the 4Ps: power (relating to uterine contractions), passage (referring to the mother’s pelvic bone structure), passenger (the baby), and fetal presentation (the position of the fetus). Hypotonic labor primarily involves a problem with ‘power’, which means that there isn’t enough force to move the baby down, dilate the cervix, and ultimately to help deliver the baby and placenta.
What Causes Hypotonic Labor?
Hypotonic labor, a condition associated with the drop in the strength of a woman’s contractions during childbirth, is caused by something called uterine inertia. This is also known as a hypocontractile or low-tension uterine dysfunction. Although we don’t know exactly why this happens, it’s often connected to:
* The uterus becoming overstretched or exhausted, which could happen with multiple pregnancies, a large baby, excess amniotic fluid, or having many previous pregnancies
* Physical interference with the function of the muscle layer in the uterus, possibly due to a fibroid, bladder or bowel distension
* The baby being positioned incorrectly or presenting in an abnormal way during labor, which can interfere with the reflex that triggers uterine contractions
* An unusual uterine position as may occur with an excessively tilted uterus
* Deformities in the uterus or disorganization of the muscle layer in the uterus, which can occur with underdeveloped uterus and extensive removal of fibroids
Also, in instances where the baby is born prematurely (before 30 weeks), the uterus’s responsiveness to oxytocin (a hormone that stimulates contractions) may not be fully developed yet.
Other general causes could include the mother being anemic or exhausted, or improper use of pain relief during labor.
Risk Factors and Frequency for Hypotonic Labor
Issues with labor are more common in women having their first child (25%) compared to women who have had children before (10%), especially those due to weak uterine contractions. The rate tends to be higher for older women having their first child. Weak contractions are a common reason for first-time Cesarean sections. Many C-sections are done because labor isn’t progressing as it should. The main reason for this is usually weak uterine contractions.
- Problems with labor are often seen in women having their first child, and are more frequent in older women having their first child.
- Weak contractions can often cause labor to progress slowly, resulting in a C-section being necessary.
- In 2017, 37% of births in the United States and 27.3% in the United Kingdom were performed by C-section.
- The increasing trend of C-section deliveries, especially in developed countries, has led to more scrutiny on the reasons for performing first-time C-sections.
Signs and Symptoms of Hypotonic Labor
During an assessment for labor, it’s confirmed that the patient is in actual labor when there’s verified documentation of the onset time. Over time, the laboring woman may feel less pain, less frequent contractions, and more time between each contraction. It’s uncommon, but some women may experience exhaustion later on due to anxiety. Generally, in early labor, the baby isn’t affected. One common sign associated with labor is that the cervix is not opening as much as it should or the baby isn’t advancing down the birth canal.
In ineffective labor, there could be slower than normal progress known as protracted labor, or no progress at all, known as the arrest of labor, or insufficient pushing effort. For any of these conditions to be diagnosed, the woman has to be in an active phase of labor with her cervix opened up to 4 cm.
Testing for Hypotonic Labor
When a woman goes into labor, doctors use tools and techniques to monitor the activity of the uterus. One common method is called palpation, where the doctor uses their hands to feel the abdomen during contractions. They can also use a special device called an external tocodynamometer to watch the contractions. This tool helps identify if the contractions are frequent and strong enough (usually expecting to see about 3 to 4 contractions every 10 minutes, each lasting 45 seconds).
Another technique is to perform a digital examination to assess the cervix (the lower part of the uterus). This allows doctors to track changes in the cervix opening (dilation), thinning (effacement), and how far the baby’s head has moved down into the birth canal (fetal station) at specific intervals during the labor process. Changes recorded over time can be plotted on a graph known as a partogram to compare with what’s typically expected during labor.
If the labor is progressing slower than expected, as shown on the partogram, the doctor would say the labor is taking longer than usual. In severe cases, if dilation doesn’t change for over 6 hours despite contractions, this is termed as stalled labor.
Another tool that can be used is called an internal pressure catheter. It measures the pressure created during each contraction, and this is recorded in a unit called Montevideo units (MVU). If the pressure is less than 200 to 250 MVUs, it’s considered insufficient and unlikely to promote dilation and descent of the baby. This tool might provide a more precise reading in women with significant obesity.
Treatment Options for Hypotonic Labor
During labor, monitoring both the mother and the baby is extremely important to make sure there are no issues that might slow down progress. This could include problems like the baby’s head being too big to pass through the mother’s pelvis (cephalopelvic disproportion, or CPD) or the baby not being in a good position for birth (fetal malpositioning). When things aren’t progressing as they should, the medical team has a range of measures—both supportive and active—they can use to help.
Supportive measures focus on keeping the mother comfortable and calm. This is important, since stress can release adrenaline, which can actually slow down contractions. Techniques might include things like encouraging the mother to walk around, optimizing her position, making sure her bladder is empty, maintaining her hydration, and offering pain relief.
Active measures are more direct interventions aimed at improving contractions.
One such method is an amniotomy, which involves breaking the amniotic sac (the water bag that the baby floats in). This procedure releases chemicals called prostaglandins which help to stimulate contractions and get labor going. An amniotomy is generally only attempted when the circumstances suggest a vaginal birth is likely—for example, when the cervix has opened up enough (over 4 cm dilation), the baby has descended low enough, and the baby’s head is well positioned.
Another important medication that can be used to boost contractions is oxytocin. However, it’s only used if there are no factors that make it unsafe for the mother or the baby. Careful monitoring of the contraction rate is needed for the safe and effective use of oxytocin.
If labor still isn’t progressing correctly despite these measures, medical staff may assist in vaginal delivery using tools like forceps, vacuum devices, or by manually turning the baby in certain cases. Additionally, if all measures fail to stimulate contractions, or if it’s found early on that the baby’s position can’t be corrected or that the mother’s pelvis is too small for a vaginal delivery, ending labor through a cesarean section (C-section) might be necessary. Other reasons for a C-section might include if the mother is too exhausted to continue with labor, if the baby is showing signs of distress, or if the cervix is not fully dilated.
What else can Hypotonic Labor be?
If a woman in labor has unusually weak contractions, often referred to as ‘hypotonic labor’, doctors could consider several possible reasons for this. These might include:
- ‘Braxton-Hicks contractions’, which are practice contractions that can occur before real labor begins
- ‘Amnionitis’, an inflammation of the amniotic sac
- Issues with the baby’s position or presentation, also known as ‘malpresentation/malpositioning’
- ‘Uterine rupture’, which is when the womb tears, a very serious but rare condition
What to expect with Hypotonic Labor
The outlook is generally positive. When a labor that is less intense than normal (hypotonic labor) is identified early and promptly managed, the health outcomes for both mother and baby tend to be good. However, if the uterus is not contracting normally and there are complications, such as the baby’s head being too big for the mother’s pelvic area (cephalopelvic disproportion), the baby not being in the correct position for birth (fetal malpositioning), or the baby showing signs of distress, then the risk of needing surgery, like a cesarean section, increases. Deciding to perform a cesarean section early on can improve the health of both mother and baby.
Possible Complications When Diagnosed with Hypotonic Labor
Hypotonic labor can lead to various complications that may affect both the mother and the baby. These can include:
- Stalled progress in labor
- Mother experiencing anxiety and exhaustion
- Heavy bleeding after childbirth due to weak uterine muscles
- The placenta remaining in the womb due to ineffective muscle retraction
- Increased likelihood of an instrumental delivery, which can potentially cause injuries to the mother or baby
- Higher probability of needing a cesarean section, which carries its own risk of surgical and anesthetic complications
- Baby experiencing distress and possibly a lack of oxygen at birth
Preventing Hypotonic Labor
Preparing mentally for childbirth seems to help manage pain better during the process. This preparation generally starts during regular pregnancy check-ups and discussions about pain relief for childbirth. Mental readiness may help lower the need for spinal or epidural numbing during childbirth. This is important because this type of numbing can sometimes lead to a slower labor due to weakened contractions.
