1. What every clinician should know
Clinical features and incidence
Meconium is the intestinal content of the fetus. It is made of water (as much as 80%), mucus, lanugo, mucopolysaccharides, bile pigments (mainly biliverdin), intestinal enzymes, and cells that have been shed from the skin and the intestinal tract.
Meconium-stained amniotic fluid at delivery occurs in 12-15% of all deliveries and is more common in term or post-term pregnancies. Its accumulation in amniotic fluid may be due to increased passage (bowel peristalsis) or decreased amniotic fluid clearance, both of which can be related to pathologic conditions, such as acute or chronic hypoxia.
Clinically, presence of meconium is suspected in the presence of greenish discoloration of amniotic fluid at membrane rupture or at transcervical amnioscopy.
Unlike meconium-stained amniotic fluid, meconium aspiration syndrome (MAS) is rare, occurring in about 2% of deliveries with meconium-stained amniotic fluid. However, it is associated with significant mortality and morbidity risk in the newborn.
Both physiological and pathological conditions can determine meconium staining of amniotic fluid. Meconium is indeed physiologically defecated in utero, especially at term and decreased fetal swallowing at term may lead to “physiologic” accumulation of meconium. Increase in bowel peristalsis is postulated to occur in the setting of intra-amniotic infection. Some pathologic processes that may interfere with amniotic fluid clearance, causing meconium accumulation in the amniotic fluid, include fetal asphyxia, intrauterine infection, and obstetric cholestasis.
Peristalsis of the fetal intestines is present as early as 8 weeks’ gestation and active absorption of glucose and amino acids by intestinal villi occurs by 10-12 weeks. Intestinal enzymes including the disaccharidases and alkaline phosphatase have been recovered from amniotic fluid specimens in the mid trimester (14-22 weeks), suggesting that there is free passage of the intestinal contents to the amniotic cavity. The anal sphincter develops at about 20-22 weeks. Obviously, control of the anal sphincter is involuntary in the newborn (and fetus). Defecation in utero, both in the early midtrimester and in the third trimester, has been documented by 3-D sonography. Meconium passage in newborn infants is a normal process typically occurring in the first 24-48 hours of life.
2. Diagnosis and differential diagnosis
There is no definitive test that confirms the cause of meconium accumulation in amniotic fluid. In cases of suspected cholestasis, serum bile acids and liver transaminases should be requested to confirm the diagnosis. Intrauterine infection can be suspected at clinical examination, in the presence of maternal fever, and elevated white blood cell count and C protein.
Presence of meconium in amniotic fluid should always heighten suspicion that the fetus is at increased risk. For this reason, if meconium is detected, continuous electronic fetal heart rate (FHR) monitoring is recommended. If the FHR pattern remains normal, meconium staining may be considered physiological: it has been demonstrated that meconium in the absence of other signs of non-reassuring fetal status is not a sign of fetal acidemia. However, thickening of meconium consistency during labor is associated with worse neonatal outcome.
If meconium is detected at amniocentesis in the context of severe cholestasis unresponsive to treatment, delivery should be considered after 34 weeks given the known association between meconium passage in cholestasis and risk of fetal death.
If moderate or thick meconium is detected during labor, amnioinfusion should be considered in settings where facilities for perinatal surveillance are limited, as it is associated with substantive improvements in perinatal outcome. Amnioinfusion does not appear to be equally effective in settings with standard peripartum surveillance perhaps because its beneficial effects are masked by other strategies to optimise neonatal outcome.
A pediatrician should be present at delivery, even if the FHR pattern is normal during labor.
Routine amnioscopy or the ultrasound detection of thick meconium have not been demonstrated to reduce the rate of adverse outcome.
4. Prognosis and outcome
The relationship between meconium passage in utero and fetal acidemia at birth is controversial. Pregnancies at term with meconium-stained amniotic fluid have been demonstrated to have a non-reassuring FHR in less than 14% of cases, 5-minute Apgar score below 7 in less than 3% of cases, and umbilical artery pH less than 7.10 in less than 4% of cases.
Some authors have demonstrated that if the FHR during labor remains normal, there are no significant differences in fetal pH between meconium-stained and non-meconium amniotic fluid, whereas if the FHR is abnormal, meconium-stained amniotic fluid is associated with a significantly higher risk of pH less than 7, Apgar score below 7, and cesarean section for non-reassuring fetal status. These risks are increased further if oligohydramnios is present.
The most severe complication of meconium-stained amniotic fluid is meconium aspiration syndrome (MAS), a clinical diagnosis that includes meconium-stained amniotic fluid at delivery, respiratory distress, and a typical appearance on chest radiograms. Its severity is very variable, ranging from transient respiratory distress requiring little therapy to severe respiratory compromise, with a mortality rate as high as 40%.
However, it is a rare complication, occurring only in about 2% of all deliveries with meconium-stained amniotic fluid. Fortunately, the treatment of severe MAS has dramatically improved during the last decade, leading to a decrease in morbidity and mortality.
What is the evidence for specific management and treatment recommendations?
Hofmeyr, GJ, Xu, H, Eke, AC. “Amnioinfusion for meconium-stained liquor in labour”. Cochrane Database Syst Rev. vol. 1. 2014 Jan 23. pp. CD000014
Ramón y Cajal, CL, Martinez, RO. “Defecation in utero: a physiologic fetal function”. Am J Obstet Gynecol. vol. 188. 2003. pp. 153-156.
Locatelli, A, Regalia, AL, Patregnani, C, Ratti, M. “Prognostic value of change in amniotic fluid color during labor”. Fetal Diagn Ther. vol. 20. 2005. pp. 5-9.
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