The appearance of meconium-stained liquor during labour is generally considered to be a sign of hypoxia and a predictor of poor fetal outcome. But is this always true?
Statistics for the presence of meconium-stained amniotic fluid vary greatly between 5% to 25% (Hirani et al. 2015). Or as Qadir et al. (2016) suggests between 1 to 18%. Yet regardless of these variable statistics, the significance of meconium in the amniotic fluid itself is also a widely debated subject.
Traditionally meconium has been considered a sign of fetal distress due to hypoxia but as Gupta et al. (2017) point out, as well as being a potential warning of fetal asphyxia, it can also simply be a physiological response of a normally maturing gastrointestinal tract, causing no ill effects on the fetus at all.
Why is Meconium Sometimes Passed Before Birth?
Reed (2015) suggests that there are three reasons why the fetus may pass meconium before birth:
Their digestive system has reached maturity and the intestine has begun working and moving the meconium out. This is the most common reason found in 15-20% of term babies and 30-40% of post-term babies.
Their cord or head is being compressed during labour causing a vagally mediated gastrointestinal peristalsis, the same reflex which causes variable heart rate decelerations. This may be why a lot of babies pass meconium as their head is compressed during the last minutes of birth.
Fetal distress resulting in hypoxia. Although the exact relationship between fetal distress and meconium-stained liquor is uncertain, it’s thought that lack of oxygen and intestinal ischaemia relaxes the anal sphincter and increases gastrointestinal peristalsis resulting in the passage of meconium.
It’s worth pointing out however, that fetal distress can be present without the presence of meconium, and meconium can be present without fetal distress.
Is the Presence of Meconium-Stained Liquor Always Dangerous?
Broadly speaking there are two classifications of meconium-stained liquor (MSL), non-significant and significant.
Non-significant MSL is defined as a thin yellow or greenish tinged fluid, containing non-particulate meconium.
Significant MSL is defined as amniotic fluid containing lumps of meconium, or dark green or black amniotic fluid that is thick and tenacious.
Additional indicators of a potentially adverse neonatal outcome include MSL associated with an abnormal cardiotocograph (CTG) which is more likely to result in caesarean section and potential neonatal complications (Priyadharshini 2013).
That said, it’s always important to consider the whole clinical picture before deciding if the presence of meconium is a significant finding or not.
Whilst it’s true that the presence of meconium in the amniotic fluid can be a potentially serious sign of fetal compromise, associated with poor perinatal outcome (Vaghela et al. 2014), it’s also true that most babies who are born in poor condition do not have meconium-stained liquor and most babies with meconium-stained liquor are born in good condition (Reed 2015).
Risk Factors for Meconium Aspiration Syndrome
In a global sense, MAS is still considered to be a marker for adverse perinatal outcomes. Whilst the passage of meconium alone is rarely a sign of significant fetal hypoxia or acidosis, meconium in the presence of an abnormal fetal heart rate (FHR) pattern should always be investigated further.
As Qadir et al. (2016) suggest the presence of thick meconium is associated with an increase in perinatal morbidity and mortality and its presence should never be overlooked.
Statistics on the incidence of MAS vary slightly from area to area but according to the Royal Berkshire NHS Foundation Trust (2017), it occurs in 1.3 out of every 1000 live-born infants and is more common in the post-term fetus.
Overall MAS is known to have a higher incidence with:
Increase in gestational age.
Birth weight >2.5 kgs.
(Hirani et al. 2015)
Low-Risk Factors for MAS
The presence of ‘thin’ or ‘non-significant’ meconium-stained amniotic fluid (MSAF) is considered by some practitioners as ‘low risk’, requiring only intermittent auscultation instead of continuous electronic fetal heart rate monitoring (Bolten and Chandraharan 2019).
High-Risk Factors for MAS
Factors associated with high risks of fetal harm include:
The presence of particulate or thick meconium.
Abnormal fetal heart rate patterns, especially tachycardia.
Absence of fetal heart rate accelerations and or, late decelerations.
Umbilical artery pH of less than 7.15.
Apgar score of less than 7 at 5 minutes.
Meconium found in the trachea.
The Debate About Intervention
Whilst each hospital will have their own documented policies on the management of meconium-stained liquor, Reed (2015) urges all practitioners to consider the holistic picture and suggests the following practical steps that might help lower the risk of meconium aspiration syndrome:
Avoid artificial rupture of the membranes (ARM) during labour. If there is meconium present it will remain well diluted and the amniotic fluid will protect the baby from compression during contractions.
Ensure that the mother knows meconium is a variation and not necessarily a complication. A post-dates baby with old meconium is very different from a 38-week baby with thick fresh meconium.
Avoid any interventions that are associated with fetal distress such as ARM, the use of syntocinon/pitocin, or directed pushing.
Better Safer Care (2018) also comment on the following areas of uncertainty in clinical practice suggesting that:
There is no evidence that management should be based on the consistency of meconium.
There is no evidence that techniques used to inhibit gasping after birth can be effective in reducing the incidence of MAS.
Perhaps more controversially Reed (2015) also suggests that many of the interventions implemented due to the presence of meconium are more likely to cause complications than the meconium itself.
For example, typical responses to meconium-stained liquor include CTG monitoring with a reduction in mobility and tightened time limits for labour, all of which could make induction, augmentation or caesarean section more likely.
Hudson (2015) takes a similar view emphasising the need to avoid stimulating babies born through MSL to avoid aspiration. Delayed cord clamping is also suggested as early clamping and cutting of the cord leaves the baby with no choice but to inhale.
Postnatal Observation and Discharge
MAS rarely develops in infants born through meconium-stained liquor with APGAR scores of 8 or more and in the view of van Ierland, de Boer and de Beaufort (2010), these infants can be safely discharged from hospital shortly after birth.
However, assessing the risk of developing MAS doesn’t stop at delivery and most units will have a policy of extended postnatal observation for the first few hours of life.
Bolten, M and Chandraharan, E 2019, 'The Significance of ‘Non-Significant’ Meconium Stained Amniotic Fluid (MSAF): Colour Versus Contents', Journal of Advances in Medicine and Medical Research, [online] pp.1-7, viewed 11 January 2020, http://journaljammr.com/index.php/JAMMR/article/view/30192
Hirani, G et al. 2015, 'A Study on Clinical Profile of Meconium Aspiration Syndrome in Relation to Gestational Age and Birth Weight and Their Immediate Outcome', RGUHS Med Sciences, 5(2), pp. 60-65 [online], viewed 7 January 2020, http://rjms.in/index.php/rjms/article/view/116869/80932