The value of customised centiles in assessing perinatal mortality risk associated with parity and maternal size.

OBJECTIVE: We wanted to compare customised and population standards for defining smallness for gestational age (SGA) in the assessment of perinatal mortality risk associated with parity and maternal size. DESIGN: Population-based cohort study. SETTING: Sweden. POPULATION: Swedish Birth Registry database 1992-1995 with 354 205 complete records. METHOD: Coefficients were derived and applied to determine SGA by the fully customised method, or by adjustment for fetal sex only, and using the same fetal weight standard. MAIN OUTCOME MEASURE: Perinatal deaths and rates of small for gestational age (SGA) babies within subgroups stratified by parity, body mass index (BMI) and maternal size within the BMI range of 20.0-24.9. RESULTS: Perinatal mortality rates (PMR) had a U-shaped distribution in parity groups, increased proportionately with maternal BMI, and had no association with maternal size within the normal BMI range. For each of these subgroups, SGA rates determined by the customised method showed strong association with the PMR. In contrast, SGA based on uncustomised, population-based centiles had poor correlation with perinatal mortality. The increased perinatal mortality risk in pregnancies of obese mothers was associated with an increased risk of SGA using customised centiles, and a decreased risk of SGA using population-based centiles. CONCLUSION: The use of customised centiles to determine SGA improves the identification of pregnancies which are at increased risk of perinatal death.

Perinatal outcome in SGA births defined by customised versus population-based birthweight standards.

OBJECTIVE: To determine whether customised birthweight standard improves the definition of small for gestational age and its association with adverse pregnancy outcomes such as stillbirth, neonatal death, or low Apgar score. DESIGN: Population based cohort study. POPULATION: Births in Sweden between 1992-95 (n = 326,377). METHODS: Risks of stillbirth, neonatal death, and Apgar score under four at five minutes were calculated for the lowest 10% birthweights according to population-based and customised standards, and were compared with the data from the group with birthweights over this limit. Population attributable risks for stillbirth using various birthweight centile cutoffs were calculated for the two standards. OUTCOME MEASURES: Odds ratios and 95% confidence intervals for stillbirth, neonatal death and Apgar score under four at five minutes, and population attributable risks for stillbirth at different birthweight centiles. RESULTS: Risks of stillbirth, neonatal death, and Apgar score under four at five minutes and population attributable risks of stillbirth were consistently higher if 'small for gestational age' was classified by a customised rather than by the population-based birthweight standard. Compared with infants who were not small for gestational age by both standards, the odds ratio for stillbirth was 6.1 (95% CI 5.0-7.5) for small for gestational age by customised standard only, whereas it was 1.2 (95 % CI 0.8-1.9) for small for gestational age by population standard only. CONCLUSIONS: Compared with the population-based birthweight standard, a customised birthweight standard increases identification of fetuses at risk of stillbirth, neonatal death and Apgar score under 4 at 5 minutes, probably due to improved identification of fetal growth restriction.

Caffeine intake and the risk of first-trimester spontaneous abortion.

BACKGROUND: Some epidemiologic studies have suggested that the ingestion of caffeine increases the risk of spontaneous abortion, but the results have been inconsistent. METHODS: We performed a population-based, case-control study of early spontaneous abortion in Uppsala County, Sweden. The subjects were 562 women who had spontaneous abortion at 6 to 12 completed weeks of gestation (the case patients) and 953 women who did not have spontaneous abortion and were matched to the case patients according to the week of gestation (controls). Information on the ingestion of caffeine was obtained from in-person interviews. Plasma cotinine was measured as an indicator of cigarette smoking, and fetal karyotypes were determined from tissue samples. Multivariate analysis was used to estimate the relative risks associated with caffeine ingestion after adjustment for smoking and symptoms of pregnancy such as nausea, vomiting, and tiredness. RESULTS: Among nonsmokers, more spontaneous abortions occurred in women who ingested at least 100 mg of caffeine per day than in women who ingested less than 100 mg per day, with the increase in risk related to the amount ingested (100 to 299 mg per day: odds ratio, 1.3; 95 percent confidence interval, 0.9 to 1.8; 300 to 499 mg per day: odds ratio, 1.4; 95 percent confidence interval, 0.9 to 2.0; and 500 mg or more per day: odds ratio, 2.2; 95 percent confidence interval, 1.3 to 3.8). Among smokers, caffeine ingestion was not associated with an excess risk of spontaneous abortion. When the analyses were stratified according to the results of karyotyping, the ingestion of moderate or high levels of caffeine was found to be associated with an excess risk of spontaneous abortion when the fetus had a normal or unknown karyotype but not when the fetal karyotype was abnormal. CONCLUSIONS: The ingestion of caffeine may increase the risk of an early spontaneous abortion among non-smoking women carrying fetuses with normal karyotypes.

Genetic influence on birthweight and gestational length determined by studies in offspring of twins.

OBJECTIVE: To determine the relative importance of genetic effects on birthweight, gestational length and small for gestational age. DESIGN: A cohort study, using individual record linkage between the population-based Swedish Twin and Birth Registers to estimate twin similarities in twins with known zygosity. POPULATION: Included were 868 monozygotic and 1141 dizygotic female twin pairs, born in Sweden before 1959, who both delivered single births from 1973-1993. METHODS: Quantitative genetic methods, offspring birthweight, gestational length and small for gestational age birth in twin sisters. MAIN OUTCOME MEASURES: Twin similarities measured as probandwise concordance rates and intra-class correlations for birthweight, gestational length and small for gestational age births. RESULTS: Concordance rates and intra-class correlations for birthweight, gestational length and small for gestational age were consistently higher in monozygotic compared with dizygotic twins. Model fitting suggested heritability estimates in the range from 25% to 40%. CONCLUSIONS: This study suggests genetic effects not only for birthweight and fetal growth, but also for gestational length. The mediation of these genetic effects may partly be due to similarities in maternal antropometric measures, lifestyle and medical complications during pregnancy. The study does not distinguish between fetal and maternal genetic effects.

Outcomes of post-term births: the role of fetal growth restriction and malformations.

OBJECTIVE: To evaluate the risks of adverse pregnancy outcomes among term and post-term small for gestational age (SGA) and appropriate for gestational age (AGA) births, before and after excluding infants with congenital malformations. METHODS: We did a population-based study of 510,029 singleton term (37-41 completed weeks) and post-term (at or after 42 weeks) births recorded in the Swedish Birth Register. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to estimate the risks of stillbirth, infant death, convulsions, meconium aspiration, and Apgar score less than 4 at 5 minutes. RESULTS: Among term births, 2.2% were SGA; among post-term births, 3.8% were SGA. Compared with term AGA births, term SGA births were at increased risk of stillbirth (OR 8.02; 95% CI 6.57, 9.80) and infant death (OR 7.57; 95% CI 6.39, 8.96). Among post-term SGA births, the ORs were 10.56 (95% CI 6.95, 16.05) for stillbirth and 5.00 (95% CI 3.04, 8.22) for infant death. When births with congenital malformations were excluded, the risk of infant death decreased considerably. Risks of convulsions and Apgar score less than 4 were higher in SGA than AGA infants. Post-term AGA infants had no significant increase in the risks of stillbirth or infant death but did have increased risks of convulsions, meconium aspiration, and Apgar score less than 4. CONCLUSION: The increased risk of stillbirth in post-term pregnancies is partly explained by an increased rate of SGA infants. The increased risk of death among SGA infants is caused to a large extent by congenital malformations.

Preterm and term births of small for gestational age infants: a population-based study of risk factors among nulliparous women.

OBJECTIVE: To study risk factors for small for gestational age (SGA) infants by gestational age among nulliparous women and to estimate mortality rates among SGA and appropriate-for-gestational-age (AGA) infants by gestational age. DESIGN: A population-based study from the Swedish Medical Birth Register. Setting Sweden 1992 1993. POPULATION: Liveborn singleton infants to nulliparous women (n = 96,662). MAIN OUTCOME MEASURES: Crude and adjusted odds ratios of risk factors for SGA by gestational age. Rates of neonatal and postneonatal mortality. RESULTS: Older maternal age (> or = 30 years) was foremost associated with increased risks of very and moderately preterm SGA (> or = 32 weeks and 33-36 weeks, respectively), but also with term SGA (> or = 37 weeks). Risks of SGA increased with decreasing maternal height at all gestational ages. Smoking increased the risks of moderately preterm and term SGA. Short maternal education increased the risk of preterm SGA and low pre-pregnancy body mass index slightly increased the risk of term SGA. Pre-eclampsia and essential hypertension foremost increased the risk of very preterm SGA (OR = 40.5 and 32.4, respectively) and moderately preterm SGA (OR = 17.4 and 10.6, respectively), but also increased the risk of term SGA. Neonatal and postneonatal mortality rates of SGA infants were substantially influenced by gestational age, and mortality rates were consistently higher among preterm SGA infants compared with AGA infants. CONCLUSIONS: Risk factors for SGA and mortality rates among SGA infants vary by gestational age. A subdivision of risk factors by gestational age adds knowledge, particularly about risks of preterm SGA, where the highest rates of mortality were observed.