The Curse of Meconium facts!!
Meconium is a mixture of mostly water (70-80%) and a number of other interesting ingredients (amniotic fluid, intestinal epithelial cells, lanugo, etc.). Around 15-20% of babies are born with meconium stained liquor.
There are three reasons (theoretically) that a baby will open his/her bowels before birth (Unsworth & Vause 2010):
Because their digestive system has reached maturity and the bowel has begun working. This is the most common reason and 30-40% of post-term babies will have passed meconium in-utero.
Because their cord or head is being compressed (during labour) ie. a vaginally mediated gastrointestinal peristalsis – the same reflex which causes variable heart rate decelerations. This is a normal physiological response and can happen without fetal distress.
Fetal distress resulting in hypoxia. However the exact relationship between fetal distress and meconium stained liquor is uncertain. The theory is that intestinal ischaemia relaxes the anal sphincter and increases gastrointestinal peristalsis = passage of meconium. However, fetal distress can be present without meconium, and meconium can be present without fetal distress.
Meconium alone cannot be relied on as an indication of fetal distress: “… meconium passage, in the absence of other signs of fetal distress, is not a sign of hypoxia…”(Unsworth & Vause 2010). An abnormal heart rate is a better predictor of fetal distress; and an abnormal heart rate + meconium provides an even better indication that a baby may be in trouble. In addition, thick meconium rather than thin meconium is associated with complications. Despite this, babies who are known to have passed meconium (of any variety) without any other risk factors are treated as if they are in imminent danger. I am guessing this is because if a previously unstressed baby becomes hypoxic during labour it may result in the dreaded MAS.
Meconium Aspiration Syndrome (MAS)
MAS is the major concern when meconium is floating about in the amniotic fluid. It is an extremely rare complication – around 2-5% of the 15-20% of babies with meconium stained liquor will develop MAS (Unsworth & Vause 2010). Of the 2-5% of the 15-20%, 3-5% of babies will die. OK enough %s of %s – basically it is very rare but can be fatal.
MAS occurs when the baby inhales meconium stained liquor during labour, birth or immediately following birth. You can see a simple explanation of MAS in utero (where it usually happens) here. However this animation does not detail why aspiration might take place.
Babies make shallow breathing movements during pregnancy. Breathing movements slow down in response to prostaglandins before birth. During labour and birth it is very unlikely that a baby will inhale liquor (and any meconium in it).
This will only happen if the baby becomes extremely hypoxic and begins to gasp in utero in an attempt to get oxygen. So, meconium alone is not a problem. Meconium + a hypoxic baby = the possibility of MAS (Davies & MacDonald 2008).
So you would think that the sensible thing to do if a baby has passed meconium (for whatever reason) is to create conditions that are least likely to result in hypoxia and MAS. This is where I get confused because common practice is to do things that are known to cause hypoxia, for example:
Inducing labour if the waters have broken (with meconium present) and there are no contractions or if labour is ‘slow’ in an attempt to get the baby out of the uterus quickly.
Performing an ARM (breaking the waters) to see if there is meconium in the waters when there are concerns about the fetal heart rate.
Creating concern and stress in the mother which can reduce the blood flow to the placenta.
Directed pushing to speed up the birth.
Having extra people in the room (pediatricians), bright lights and medical equipment which may stress the mother and reduce oxytocin release.
Cutting the umbilical cord before the placenta has finished supporting the transition to breathing in order to hand the baby to the pediatrician.