There were 12,886 twins in the study. The risk of low Apgar score was higher in twins than in singletons, and more pronounced for the second twin (OR 4.14, CI 3.52, 4.87) than for the first (OR 2.40, CI 1.97, 2.93), after standardization for birth weight, 2.41 (2.01, 2.89) and 1.62 (1.31, 2.00), respectively. Singleton infants with breech presentation had OR 2.57 (CI 2.22, 2.98) for low Apgar score at 5 minutes, compared with infants in vertex presentation. For vaginally born infants, the OR for low Apgar score in breech deliveries compared with vertex deliveries was 6.67 (CI 5.87, 7.58). Epidural analgesia was associated with an increased risk of low Apgar score, with an OR of 2.10 (1.94, 2.29) for vaginally delivered singleton infants in vertex presentation.
In June, July, August, and December, the risk of low Apgar score was higher than in the rest of the year, but only in August the increased risk was significant (OR 1.10, CI 1.02, 1.18). The risk of low Apgar score was higher in infants born during “non-office hours” (5.00 PM–6.59 AM) as compared with infants born during office hours (8.30 AM–3.59 PM) (OR 1.21, CI 1.14, 1.29). The risk during intermediate hours did not differ significantly from the other two groups (OR 1.05, CI 0.95, 1.16).
The infant mortality was 48 per 1000 (371 of 7787) in infants with 5-minute Apgar score under 7, and 107 per 1000 (230 of 2155) with Apgar score under 4, compared with 1.8 per 1000 in those with 5-minute Apgar score 7 or more (OR 14.4, CI 12.5, 16.5). The neonatal mortality was 40, 95, and 0.6 per 1000, respectively, and the first week of life 34, 86, and 0.3 per 1000, respectively. Infants who had an Apgar under 7 at 5 minutes and also under 7 at 10 minutes had an infant mortality rate of 129 per 1000 (239 of 1849).
Of infants who had Apgar scores below 7 at 5 minutes, 64 per 1000 (498 of 7787) had seizures as compared with 0.9 per 1000 (904 of 1,020,825) in infants with Apgar 7 or more (RR 71.5, CI 64.2, 74.5). Among infants with Apgar score below 7 at 5 minutes who also had Apgar score below 7 at 10 minutes, the rate of seizures was 149 per 1000 (283 of 1895). Of infants with an Apgar score under 7 at 5 minutes, 11.4 per 1000 (89 of 7787) had intracranial hemorrhage, compared with 0.27 per 1000 (273 of 1,020,825) in infants with an Apgar 7 or more at 5 minutes (RR 42.4). In infants with Apgar score under 7 at 5 minutes and also under 7 at 10 minutes, 31 per 1000 (50 of 1612) had intracranial hemorrhage during the neonatal period.
By linking the Medical Birth Registry to the Registry of Hospital Discharges, we investigated the risk of long-term sequels for infants with a 5-minute Apgar score under 7 (Table 3). The analyses were stratified for birth year, maternal age, parity, and smoking. The OR for developing cerebral palsy was 31.4 (CI 27.3, 36.1), for epilepsy 7.9 (CI 6.6, 9.4), for “seizures” (including febrile convulsions) 1.6 (CI 1.4, 1.9), and for mental retardation 9.5 (CI 7.2, 12.5).
The Swedish Medical Birth Registry, providing data on 98–99% of births in Sweden, is one of the most complete birth registers in the world. We used the register to perform a population-based study on term infants with low Apgar score at 5 minutes after birth.
A low Apgar score is not a specific indicator of birth asphyxia because there may be other causes of this depression at birth. In this study, we excluded the two most apparent, prematurity and severe malformations. Biochemical markers, measuring acidemia, provide more specific evidence of intrauterine hypoxia, but are not available in the register. On the other hand, the Apgar score provides information about compromise of the neonate, which is not obtainable by biochemical measurements. The diagnosis “birth asphyxia” in ICD-9 and ICD-10 is based on a low 1-minute Apgar score, which often is caused by a temporary depression. A 10-minute Apgar score is often lacking in the Medical Birth Registry when a 5-minute Apgar score is 10, and does not always reflect intrapartum events. Thus, to investigate the 5-minute Apgar score may be as close we can come to “asphyxia” in a register study.
The incidence of 5-minute Apgar score under 7 in term infants increased from 1992 to 1997 (after a preceding decrease during 1988–1992). Multiple birth, immigration, and epidural analgesia, three potential risk factors for low Apgar score, which all had increased during 1992–97, may have contributed to the increase.
Many studies on asphyxia are based on data from 1-minute Apgar score,6 and some include preterm infants,7 making comparisons difficult. A study from western Sweden showed a rate of 6.9 per 1000 for 5-minute Apgar score under 7 in term infants during 1985–91,8 comparable with the lowest annual rate in our study. In England, the incidence of “clinically significant birth asphyxia” (post-asphyxic encephalopathy) was 6 per 1000, with no changes in incidence over a 10-year period.9 Our incidence of Apgar score below 4 at 5 minutes was comparable with an earlier report from Sweden.10 However, the perinatal mortality rate of 210 per 1000 during 1973–1979 in those infants was much higher than the mortality before 7 days in our study, 86 per 1000 (stillbirths excluded). In Sweden, like in other western countries, the perinatal mortality rate is still decreasing. The official perinatal mortality rate in Sweden decreased from 14 per 1000 in 1973 to 5 per 1000 in 1998.
It has been debated whether a low Apgar score and asphyxia are related to subsequent cerebral palsy.6,11,12 The OR for having cerebral palsy, for an infant with 5-minute Apgar score under 7, was in our study as high as 31, when calculated from hospital diagnoses. We only estimated the OR because the Hospital Discharge Registry, which includes only in-patients, might underestimate the incidence.
Hypoxic ischemic encephalopathy, a consequence of birth asphyxia, was included in the register only since 1997 (ICD-10). However, neonatal seizures, a symtom related to moderate and severe hypoxic ischemic encephalopathy,13 and known to be a risk factor for later neurological sequele,14 occurred in 6% of infants with Apgar score under 7 at 5 minutes (RR 71.5, CI 64.2, 74.5), and in 15% (283 of 1895) of those with Apgar score below 7 also at 10 minutes.
We found an increased risk of being born with a low Apgar score during “non-office hours” (OR 1.2), as well as during the periods of general holiday (OR 1.1). This was in accordance with a Welsh study, where the RR for infant death was 2.18 for infants delivered 9 PM–8.59 AM and 1.99 during July and August.15 This may suggest that the personnel resources (quantitatively or qualitatively) may be a factor of importance.
In the present study, as in a previous,16 smoking was found to increase the risk of a low 5-minute Apgar score, particularly in older multiparas. Confounding factors for smoking are social class, alcohol intake, and drug abuse, which could not be investigated.
Our study supported the results of another large register study reporting higher rates of asphyxia among the neonates of mothers beyond age 40 and in primiparas women.17 In the present study, the age-related risk was moderate. The risk increment is probably multifactorial. Several complications of pregnancy increase with maternal age.17,18 Some complications, such as preeclampsia, predominantly affect the first pregnancy. Women with severe complications during their first pregnancy are also more likely to receive optimal care during subsequent pregnancies and to have fewer children.
An important finding in this study was the strong influence on low Apgar score by birth weight and gestational age. A low birth weight is known to be a risk factor for fetal compromise, being a typical finding in cases of chronic placental insufficiency,19 whereas fetal macrosomia has not gained the same interest. We found that birth weight deviation in either direction was accompanied with a similar increased risk of a low 5-minute Apgar score. The risk of fetal compromise in post-term pregnancies is in agreement with previous reports.20,21 In our study, the risk was obvious in week 41 and very pronounced in week 43.
A recent study showed an increased risk of perinatal morbidity and mortality in planned vaginal breech delivery compared with planned cesarean delivery in breech presentation.22 A Danish study showed a 15-fold risk of low Apgar score in vaginal breech delivery.23 In a study from our department, vaginal breech delivery was the strongest among examined risk factors for acidemia at birth.24 The present study supported these results, showing an increased risk for a low 5-minute Apgar score in breech presentation, mainly in vaginal delivery (OR 6.67), probably reflecting increased risks of both asphyxia and trauma. Twin birth was also a risk factor for low Apgar score, twice as high in the second born twin, also after adjusting for birth weight. Problems during delivery for the second born twin are likely, but also the possibility of a delay in delivery of a twin already compromised in utero.
We conclude that 5-minute Apgar score under 7 in term infants is associated with an increased risk of neonatal morbidity, infant mortality, and neurologic impairment. In this large population-based register study of over 1 million term births, it is not possible to investigate birth asphyxia. However, the outcome indicates that continued efforts to reduce the rate of low Apgar score are strongly indicated.
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© 2001 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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