Chapter 44 Perinatal mortality, birth asphyxia and cerebral palsy
Background facts for live-born babies
• In 2007, 92.1% of live-born babies had a birthweight in the range of 2500–4499 g. The average birthweight of live-born babies in Australia in 2007 was 3374 g. For singletons, the mean gestational age was 38.9 weeks, compared with 35.3 weeks for twins and 31.3 weeks for triplets.
• 8.1% were preterm (<37 weeks gestation), compared with 7.3% in 1997. The mean gestational age for all preterm births was 33.2 weeks. 13.7% of babies of Aboriginal and Torres Strait Islander mothers were born preterm.
• 6.2% of live-born babies were of low birthweight (<2500 g). A baby may be small due to being born early (preterm), or may be small for its gestational age (intrauterine growth restriction). Low birthweight babies have a greater risk of poor health and dying, require a longer period of hospitalisation after birth, and are more likely to develop significant disabilities. The proportion of low birthweight in live-born babies of Aboriginal and Torres Strait Islander mothers was 12.5%, twice that of babies of non-Indigenous mothers (5.9%).
Definitions
• Intrauterine fetal death (IUFD). This is death of a fetus in utero after 20 weeks gestation or at birth weighing at least 400 g.
• Intrapartum death. This is fetal death during labour. If a baby is born without signs of life, but also without maceration, there is a strong presumption that death occurred during labour. There are exceptions in both directions, which require judgment on the timing of death in relation to the presumed onset of labour.
• Stillbirth (fetal death). This is the death of a fetus before delivery when the gestation has reached at least 20 weeks or the weight is over 400 g. This Australian definition differs from the World Health Organization definition, which requires the stillborn infant to weigh at least 1000 g or to have reached at least 28 weeks gestation. Therefore, caution is required in comparing international rates. The stillbirth rate is 7.4 per 1000 births in Australia. Low birthweight occurred in 79% of stillborn babies, with 28% of these unexplained. The mean gestational age of stillborn babies was 27.4 weeks in 2007 compared with 38.9 weeks for live-born babies. Preterm birth occurred in 80.8% of stillborn babies, compared with 7.6% of live-born babies.
• Neonatal death. This is the death of a live-born baby of at least 20 weeks gestation or 400 g in weight within 28 days of delivery. In comparison, the WHO definition is the death of an infant of at least 1000 g or 28 weeks gestation that occurs within 7 days of birth. In 2007, the neonatal death rate (NDR) was 2.9 per 1000 live births. Congenital abnormality occurred in a higher proportion of neonatal deaths (31.3%) than fetal deaths (20.8%). Among neonatal deaths, congenital abnormalities accounted for 67.7% of babies born at 32–36 weeks and 37.3% of babies at 37 weeks or more. The second most common cause of fetal deaths was maternal conditions (18.1%) and the proportion was highest among babies of 20–27 weeks (22.4%). Spontaneous preterm birth was a common cause of neonatal death for babies born at 20–27 and 28–31 weeks.
• Perinatal mortality rate (PMR). This is the total number of stillbirths and neonatal deaths per 1000 total births. In 2007, the PMR was 10.3 per 1000 births. Of these, 72% were fetal deaths. The PMR of babies born to Aboriginal or Torres Strait Islander mothers remains almost twice that of babies born to other mothers (20.1). PMR varied by sociodemographic, maternal and pregnancy risk factors. Young maternal age, maternal Indigenous status and multiple gestation were associated with higher rates of perinatal deaths. From data provided by four Australian states, the main cause of perinatal death was congenital abnormalities at 20–21 weeks gestation (38.5%). The leading cause of death at 22–27 weeks gestation was the category of maternal conditions (21.5%). Perinatal deaths of babies at 28–31 weeks, 32–36 weeks and 37–41 weeks were most commonly due to unexplained antepartum death. Maternal conditions accounted for 13.8% and spontaneous preterm birth for 10.8% of reported causes of perinatal death.
Stillbirth
• The investigation of a death should include a complete autopsy and detailed examination of the cord and placenta, and other investigations according to the clinical problem.
• Fetal death after the onset of labour has decreased by two-thirds. Antepartum deaths decreased to a lesser extent (46%) and currently make up approximately 65% of all fetal deaths. These patterns are similar in most developed countries and have resulted in a focus of attention towards reducing the antepartum stillbirth rate.
• In Australia over the past two decades, there has been no reduction in the rate of stillbirth. Stillbirths account for 70% of perinatal deaths, with approximately six babies each day stillborn. Stillbirth rates for Indigenous women are over twice that for non-Indigenous women.
Unexplained stillbirth
Birth asphyxia
Indicators
• Obstetric indicators of asphyxia may include an abnormal cardiotocograph (CTG), fetal acidosis, and the presence of meconium liquor. The latter occurs in 0.5%–20% of all births and alone is an inadequate marker of perinatal asphyxia.
• CTG is a poor predictor of birth asphyxia. Although the false-negative CTG as a predictor of fetal acidosis is below 2%, the false-positive rate may be as high as 50%.
• Neonatal indicators are the Apgar score, delay in breathing, and hypoxic ischaemic encephalopathy. Apgar scores are poor predictors of birth asphyxia. Long-term studies on babies and children have shown that, although babies with very low Apgar scores (a score <3 at 5 minutes) have an increased risk of cerebral palsy, most infants with low Apgar scores do not develop cerebral palsy. Conversely, 75% of children with cerebral palsy have Apgar scores ≥7 at 5 minutes.
Cerebral palsy
Definition. Cerebral palsy covers a range of neurological impairments, characterised by abnormal control of movement or posture resulting from abnormalities in brain development or an acquired non-progressive cerebral lesion.
Aetiology
• Although the patterns of pathological response to brain injury are similar in all cases, it is usually impossible to determine the exact time of brain injury.
• Studies have claimed that at least 90%–94% of cerebral palsy cannot be related to intrapartum hypoxia.
• Other associated factors include fetal vascular events, intrauterine infective causes (rubella, cytomegalovirus, toxoplasmosis, listeriosis), genetic causes (chromosomal abnormalities, X-linked disorders), metabolic disorders (iodine deficiency), and lead and mercury toxicity. Postnatal cerebral palsy may be caused by meningitis, near-drowning episodes or complications of prematurity.
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