Maternal labour pain and stress are associated with progressive fetal metabolic acidosis. Systemic opioid analgesia does little to mitigate this stress, but opioids readily cross the placenta and cause fetal-neonatal depression and impair breast feeding. Pethidine remains the most widely used, but alternatives, with the possible exception of remifentanil, have little more to offer. Inhalational analgesia using Entonox is more effective and, being rapidly exhaled by the newborn, is less likely to produce lasting depression. Neuraxial analgesia has maternal physiological and biochemical effects, some of which are potentially detrimental and some favourable to the fetus. Actual neonatal outcome, however, suggests that benefits outweigh detrimental influences. Meta-analysis demonstrates that Apgar score is better after epidural than systemic opioid analgesia, while neonatal acid-base balance is improved by epidural compared to systemic analgesia and even compared to no analgesia. Successful breast feeding is dependent on many factors, therefore randomized trials are required to elucidate the effect of labour analgesia.
Introduction
A number of widely held misconceptions about the effects of maternal anaesthesia and analgesia on the fetus have for many years hampered best practice in the UK. For example, despite old and new evidence to the contrary, it is commonly believed that:
- 1.
Unmodified labour is relatively harmless to the baby, and any pharmacological form of analgesia must have adverse effects.
- 2.
Epidural analgesia, the most invasive form of pain relief in labour, must also be the most damaging to the fetus.
- 3.
Pethidine should be avoided in the last hour of labour, to avoid neonatal sedation.
I cannot stress too strongly: these are indeed misconceptions, but they die hard. One common problem is that, instead of examining the effect of any procedure on the newborn, various surrogate outcomes such as maternal blood pressure, fetal heart rate, duration of labour, need for oxytocin, delivery type and even maternal fever are assumed to equate with fetal/neonatal welfare. This is erroneous. Another surrogate outcome that has in the past received too much attention is measurement of drug concentration rather than drug effect. There are several genuine yardsticks of neonatal welfare, for example: Apgar score, neurobehavioural score, acid-base balance and feeding. Although of variable reliability, all merit attention. Sadly, many studies of labour analgesia fail to include any measures of neonatal outcome.
Maternal analgesia in and around parturition may have direct pharmacological effects on the baby, because of placental transfer of maternally administered drugs, or indirect effects secondary to physiological or biochemical changes in the mother. These disparate means of affecting the fetus must be borne in mind when considering each type of procedure.
It is important that all those wishing to keep mothers correctly informed should be fully conversant with the evidence, so as to be competent to disabuse both fellow care-givers and lay consumers of their misconceptions.
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