Women heterozygous for mutations at the ornithine transcarbamylase (OTC) locus may be at risk for hyperammonaemia and its untoward effects including coma and death in the postpartum period. We present the case of a pregnant woman heterozygous for OTC deficiency (McKusick 311250) whose past medical history was significant for two prior pregnancies complicated by postpartum hyperammonaemic coma. In the index pregnancy, increased levels of serum ammonium were noted during labour. Postpartum hyperammonaemia was averted by administration of oral sodium benzoate. Our experience demonstrates that in women at risk, perilous hyperammonaemia can be prevented through appropriate medical management.