Maternal pyrexia, a persistent occipitoposterior position, and an acute intrapartum event were all labour related events associated with a significantly increased risk of newborn encephalopathy (table ). Only nine of the 18 affected infants and none of the nine control infants whose mothers had experienced pyrexia had a pathogenic organism isolated from mother or baby. A prolonged interval from rupture of membranes to delivery, abnormalities in blood pressure, a nuchal cord, cord prolapse, and shoulder dystocia were associated with a non-significantly increased risk.
Risk factors for newborn encephalopathy present in intrapartum period and adjusted for factors before birth and antepartum
Onset of labour and final mode of delivery
The final mode of delivery is determined by the delivery plan and response to intrapartum events. As the delivery plan could not be determined onset of labour was investigated as a surrogate (table ). The same proportion of cases and controls had spontaneous onset of labour. More case infants than control infants, however, were induced and fewer case infants were delivered by caesarean sections without labour.
Overall, a similar proportion of case and control infants were delivered by caesarean sections (23% (38) and 24% (96), respectively). Relative to spontaneous vaginal delivery, instrumental vaginal delivery and emergency section were associated with over a twofold increased risk of encephalopathy. Only 2.4% (four) affected infants compared with 14.5% (58) of control infants were delivered by elective section, defined as one planned at least 24 hours before the procedure (adjusted odds ratio relative to spontaneous vaginal delivery 0.17; 95% confidence interval 0.05 to 0.56). This inverse relation was not explained by social factors, including health insurance status, as these had been adjusted for. The documented indications for elective sections among case and control infants are shown in table ; previous caesarean section was the most common.
Indications for elective caesarean section documented by midwife according to whether baby had newborn encephalopathy (cases) or not (controls)
To ascertain whether different risk factor profiles explained the differences in proportion of emergency and elective caesarean sections, 14 practising consultant obstetricians from Perth were asked to develop a set of criteria which would lead them to recommend an elective section at term in the interest of the baby. The consensus, which was developed without knowledge of the study results, comprised intrauterine growth restriction, malpresentation, abnormal antepartum cardiotocography, two previous sections, macrosomia with diabetes or gestational diabetes, active herpes, and a previous difficult labour. When we applied these consensus criteria to mothers of case and control infants (table ) eligible mothers of case infants were 24 times less likely (unadjusted odds ratio relative to spontaneous vaginal delivery 24.2; 6.61 to 90.1) than eligible mothers of control infants to have been sectioned electively. Nearly 40% of the eligible case infants were eventually delivered by an emergency section and nearly 20% were delivered instrumentally or by vaginal breech delivery. The consensus criteria met by eligible mothers are summarised in table . This shows that even in the group that met the consensus criteria there was a difference in antepartum risk factor profiles between cases and controls.
Details of onset of labour and final mode of delivery in cases (babies with newborn encephalopathy) and controls by eligibility for elective caesarean section according to consensus criteria.* Values are numbers (percentages) of subjects
Consensus criteria met by mothers of cases (babies with newborn encephalopathy) and controls eligible for elective caesarean section.* Values are numbers (percentages) of subjects
Other intrapartum factors
The presence of an abnormal intrapartum cardiotocogram, meconium stained liquor, and fetal distress are usually considered to reflect intrapartum hypoxia and were not included in the adjusted analyses as they were likely to be along a causal pathway for, or the first signs of, newborn encephalopathy or were markers of encephalopathy. Inclusion of these variables in the adjusted analysis would have masked the effects of other variables that were working through them. Half the affected infants had intrapartum cardiotocography performed compared with 30% of control infants. The cardiotocogram was described as abnormal in 61% of affected infants compared with 37% of control infants (unadjusted odds ratio 4.43; 1.81 to 10.85). Meconium was described more commonly in case infants than control infants (33% v 12%; 3.72; 2.33 to 5.95) and grade III meconium in particular was much more common in case infants (13% v 1.0%; 16.7; 5.76 to 50.0). Finally, fetal distress during labour was recorded by the midwife more often in case infants than control infants (21% v 8%; 3.16; 1.84 to 5.43). For the same reason we did not include immediate characteristics of the newborn (table ) in the adjusted analysis.
Immediate characteristics of babies with encepalopathy (cases) and controls. Values are numbers (percentages) of subjects
Contribution of possible intrapartum hypoxia
In an attempt to estimate the proportion of infants who had been exposed to possible intrapartum hypoxia we used the following modified criteria: presence of an abnormal intrapartum cardiotocogram or abnormal fetal heart rate on auscultation or fresh meconium in labour, or both, together with a 1 minute Apgar score of less than 3 and a 5 minute Apgar score of less than 7.9
Cord pH measurements were not included because they were performed so infrequently. Thirty one affected infants (19%) and two control infants (0.5%) fulfilled these criteria. A further 16 cases did not strictly fulfil the definition, but there was evidence that they had experienced a significant intrapartum event which may have been associated with intrapartum hypoxia (for example, breech presentation, birth before arrival at hospital, head stuck, Apgar scores not measured). Therefore, a total of 47 case infants (29%) had evidence of having experienced intrapartum hypoxia. Only seven of these (4% of all cases), however, fulfilled the criteria of possible intrapartum hypoxia in the absence of preconceptional or antepartum abnormalities. Four case infants (2%) had no recognised antepartum risk factors or evidence of intrapartum hypoxia and 113 (69%) had only antepartum factors identified (figure ). Only 15 of these 47 case infants met the consensus eligibility criteria for an elective caesarean section.
Figure 1 Distribution of risk factors for newborn encephalopathy