Meconium-stained amniotic fluid (MSAF) occurs when a fetus passes its first stool, known as meconium, into the surrounding amniotic fluid before birth. Meconium is a thick, dark, sticky substance. While meconium is normally passed shortly after birth, its presence in the amniotic fluid during labor and delivery is common, occurring in approximately 12% to 20% of all deliveries.
Understanding Meconium-Stained Amniotic Fluid
Meconium is composed of elements swallowed by the fetus, including amniotic fluid, mucus, lanugo (fine body hair), and shed intestinal cells. Normally, this waste material remains within the baby’s intestines until after delivery. Amniotic fluid, the protective liquid surrounding the fetus, is usually clear or pale yellow.
When meconium is passed into this fluid, it changes the fluid’s color, giving it a green or brownish-green tint. The consistency can vary from thin and watery with only a slight discoloration to thick and particulate, resembling a pea soup. Healthcare providers identify MSAF during labor when the amniotic sac ruptures, or membranes are artificially broken.
Why MSAF Occurs
The passage of meconium before birth can be attributed to several factors, often related to fetal maturity or a response to stress. In many instances, especially in pregnancies extending beyond the estimated due date, the fetal gastrointestinal system matures, leading to normal intestinal peristalsis and relaxation of the anal sphincter. This physiological maturation accounts for a significant portion of MSAF cases, with incidence increasing to approximately 27% in post-term gestations.
At other times, the passage of meconium can be a response to fetal stress or distress, such as reduced oxygen flow to the fetus. Hypoxic events can lead to increased intestinal movement and anal sphincter relaxation, resulting in meconium release. Other factors linked to this occurrence include placental insufficiency, maternal high blood pressure, preeclampsia, low amniotic fluid volume (oligohydramnios), or an intrauterine infection. However, meconium passage is not always a sign of fetal distress and often has a benign cause.
Potential Effects on the Newborn
The primary concern with meconium-stained amniotic fluid is Meconium Aspiration Syndrome (MAS), which develops if the newborn inhales the fluid into their lungs. This aspiration can occur before, during, or immediately after birth, especially if the fetus gasps due to distress. MAS complicates approximately 3% to 9% of deliveries involving MSAF.
When meconium enters the lungs, it can cause several issues. It may physically obstruct the airways, leading to complete or partial blockage (atelectasis). Partial obstruction can trap air, causing over-expansion of lung tissue and potentially leading to air leaks like a pneumothorax. Meconium also triggers a chemical pneumonitis, an inflammatory reaction that irritates the airways and reduces lung compliance.
Furthermore, meconium can inactivate pulmonary surfactant, a substance necessary for proper lung function. This inactivation can lead to widespread lung collapse and impaired gas exchange. In severe cases, MAS can result in persistent pulmonary hypertension of the newborn (PPHN), a condition where blood vessels in the lungs remain constricted, impeding proper blood flow and oxygenation. Although many babies with MAS recover fully, some may experience respiratory distress, requiring medical support.
Care and Management
When meconium-stained amniotic fluid is present, a healthcare team with full resuscitation capabilities, including endotracheal intubation skills, is prepared for the delivery. Upon birth, the newborn’s vigor is immediately assessed, observing their respiratory effort, heart rate, and muscle tone.
For vigorous newborns who have strong breathing efforts, a heart rate above 100 beats per minute, and good muscle tone, routine intubation and deep tracheal suctioning are not recommended. These infants can stay with their parent for initial newborn care. However, if a newborn is non-vigorous—exhibiting depressed breathing, poor muscle tone, or a heart rate below 100 beats per minute—initial resuscitative steps are initiated under a radiant warmer.
For non-vigorous infants, gentle suctioning of the mouth and nose may be performed. If meconium is observed below the vocal cords, tracheal intubation and suctioning may be considered, but routine deep suctioning is no longer advised. Positive pressure ventilation should be provided without delay if the baby is not breathing effectively or has a low heart rate. Newborns are continuously monitored for signs of respiratory distress, such as rapid breathing, grunting, or a bluish discoloration of the skin. Depending on the severity of respiratory issues, interventions may include supplemental oxygen, mechanical ventilation, or specialized treatments like surfactant therapy or inhaled nitric oxide for pulmonary hypertension.