Methods for the evaluation of social effects on birth weight-- experiences with Swedish population registries

All births in Sweden in 1986, registered in the Medical Birth Registry, were analyzed with linked data from the November 1985 census in order to identify variables which could be used as proxy variables for the socio-economic situation of the women, using birth weight as outcome variable. Two phenomena appear to be strong socio-economic indices of significance for birth weight: whether the woman co-habited or not at the census and whether she had a long education (> 14 years) and an "academic" work. The two variables interacted, however: when the woman had a long education, cohabitation status played no significant role. Maternal education as judged from occupation seemed to play a more important role than paternal education or job. To some extent, the effects of the socio-economic variables could be explained by different smoking habits but a residual effect existed which may have different explanations. Cohabitation status and education may be used as estimates of socio-economic level of significance for birth weight and perhaps other pregnancy outcome in the analysis of, for instance, the effect of various occupational exposures on pregnancy outcome.

Secular trends in the effect of socio-economic factors on birth weight and infant survival in Sweden

Time trends in the effect of socio-economic factors on low birth weight, stillbirth, perinatal deaths and deaths up to the age of one were studied using a medical birth registry linked to census information from 1975, 1980, and 1985. For each census year, delivery outcome the following year was studied. Two socio-economically different groups of women were studied, defined by occupation/education, cohabitation, and citizenship--one privileged and one under-privileged group. A difference in birth weight distribution was found between the two groups which is only partly explainable by different smoking habits in early pregnancy and did not substantially change during the ten year observation period. In 1976, there was virtually no difference in infant mortality between the two groups. In 1981 and 1986, infant mortality had decreased in both groups but more strongly so in the privileged group, and a difference between the groups therefore appeared.

Maternal and perinatal risk factors for Wilms' tumor: a nationwide nested case-control study in Sweden

This report describes maternal and perinatal risk factors for Wilms' tumor analyzed in a case-control study nested in a nationwide cohort in Sweden. The Swedish National Cancer Registry ascertained 110 cases from among successive birth cohorts from 1973 through 1984, identified by the Swedish Medical Birth Registry, the latter based on medical records. From the Birth Registry, we matched 5 controls without cancer to each case by sex and date of birth. Wilms'-tumor children were more likely to have mothers who had been exposed to penthrane (methoxyflurane) anesthesia during delivery than mothers of controls (odds ratio (OR) = 2.4; 95% confidence interval (CI) 1.1 to 5.1); this excess risk was higher in females than males and increased with age at diagnosis. Wilms'-tumor cases were also more likely to have had physiologic jaundice (OR = 2.3; 95% CI 1.1 to 5.0). Higher parity of the mother decreased the risk of Wilms' tumor among females (OR = 0.7; 95% CI 0.5 to 1.0). We were unable to confirm the reported increased risks of Wilms' tumor for those with high birth weights or with a maternal history of hypertension or fluid retention during pregnancy, nor did we find any association with mother's age at delivery, previous stillbirth, previous live birth, gestational length or height of the child.

A registry study of very low birthweight liveborn infants in Sweden, 1973-1988

Very low birthweight liveborn infants (less than 1,500 g, VLBW) born in Sweden 1973-88 were identified from the Medical Birth Registry and efforts were made to remove wrongly recorded birthweights--9% of infants with a registered birth weight below 1,500 g were removed. Some VLBW infants were not recorded in the register and the estimate of the prevalence at birth of VLBW infants is therefore slightly underestimated. It increased from about 5.5 per 1,000 during the period 1973-84 to 6.7 per 1,000 during 1987-88. 18% of VLBW infants were involved in multiple births. Median Apgar score at 5 min increased for each 100 g birthweight class. Even at a birthweight between 1,400 and 1,499 g, a low Apgar score at 5 min was seen in 20%. The rate of cesarean section increased between 1973 and 1983 from about 10% to 60%. One-year survival for infants with a birthweight less than 1,000 g increased from less than 20% in 1973-75 to 50% in the 1986-88 cohorts. Corresponding figures for infants with a birthweight between 1,000 and 1,499 g were 60% and 90%. A markedly better one-year survival is already evident in the 600-699 g class. On stratifying for 100 g birthweight class, perinatal death risk was higher in boys than in girls and higher in multiple births than in singletons. An increased rate of congenital malformations was seen in the 1,000-1,499 g class but not in the less than 1,000 g class.

Maternal smoking in pregnancy: does it increase the risk of childhood cancer?

Experimental studies show that some compounds in tobacco smoke are transplacental carcinogens, but epidemiological data on maternal smoking and childhood cancer are inconclusive. Using the national Swedish Medical Birth and Cancer Registries, the incidence of cancer was followed through 1987 in a cohort of 497,051 children born 1982-1987 for whom information was available on maternal smoking at 2-3 months of pregnancy. A total of 327 cancers appeared including 198 solid tumours and 129 cancers of the lymphatic and haematopoietic system. The overall relative risk for cancer in children with mothers reporting smoking during pregnancy was 0.99 (95% confidence interval (CI): 0.78-1.27). Corresponding risks for solid tumours and cancers of the lymphatic/haematopoietic system were 0.96 (0.70-1.32) and 1.04 (0.71-1.52), respectively. There was no consistent increase in risk for cancer of different sites or in relation to number of cigarettes smoked per day.

An update of the Swedish reference standards for weight, length and head circumference at birth for given gestational age (1977-1981)

An update of the Swedish reference standards for weight, length, and head circumference at birth, for each week of gestational age, is presented. It is based on the total Swedish cohorts of infants born 1977-1981 (n = 475,588). A "healthy population" (79%) was extracted, using prospectively collected data. Weekly (28-42 weeks) grouped data for length and head circumference were well approximated by the normal distribution, but the distributions for birthweight were positively skewed. The original skewed distributions for birthweight were transformed, using the square root, resulting in distributions close to the Gaussian. For smoothing purposes, the weakly values for the mean and the standard deviation were both fitted by a third degree polynomial function. These functions also make possible the calculation of the continuous variable, standard deviation score, for individual newborn infants as well as a comparison of distributions between groups of infants. The reference values and charts presented here have two major advantages over the current Swedish ones: the sample size used is now sufficiently large at the lower gestational ages, so that empirically found variations can be used, and the skewness of the birth weight distribution has been taken into account. The use of the reference standards presented here improves and facilitates evaluation of size deviation at birth.

Maternal and perinatal risk factors for childhood leukemia

This report describes an exploratory population-based study of maternal and perinatal risk factors for childhood leukemia in Sweden. The Swedish National Cancer Registry ascertained 411 cases in successive birth cohorts from 1973 through 1984 recorded in the Swedish Medical Birth Registry. Using the latter, we matched five controls without cancer to each case by sex and month and year of birth. Mothers of children with leukemia were more likely to have been exposed to nitrous oxide anesthesia during delivery than mothers of controls [odds ratio (OR) = 1.3; 95% confidence interval (CI) = 1.0, 1.6]. Children with leukemia were more likely than controls to have Down's syndrome (OR = 32.5; 95% CI = 7.3, 144.0) or cleft lip or cleft palate (OR = 5.0; 95% CI = 1.0, 24.8); to have had a diagnosis associated with difficult labor but unspecified complications (OR = 4.5; 95% CI = 1.1, 18.2) or with other conditions of the fetus or newborn (OR = 1.5; 95% CI = 1.1, 2.1), specifically, uncomplicated physiological jaundice (OR = 1.9; 95% CI = 1.2, 2.9); or to have received supplemental oxygen (OR = 2.6; 95% CI = 1.3, 1.3, 4.9). Because multiple potential risk factors were analyzed in this study, future studies need to check these findings. We did not confirm the previously reported higher risks for childhood leukemia associated with being male, having a high birth weight, or being born to a woman of advanced maternal age.

Storage of saline-adenine-glucose-mannitol-suspended red cells in a new plastic container: polyvinylchloride plasticized with butyryl-n-trihexyl-citrate

Blood collection and component preparation have been performed in integrally connected multiple plastic containers made with a new plastic. This polyvinylchloride (PVC) container plasticized with butyryl-n-trihexyl-citrate (BTHC) is a new material for blood storage; it contains no di(2-ethylhexyl)phthalate (DEHP). After removal of plasma and buffy coat, the red cells were suspended in saline-adenine-glucose-mannitol (SAGM) medium. After 42-day refrigerator storage, the total adenine nucleotide concentration remained the same as the initial concentration in the red cells, whereas ATP levels had decreased to 61 percent of the initial value. The 2,3 DPG concentration was 62 percent of normal on Day 7 and 21 percent on Day 14. Glucose consumption, lactate production, potassium leakage from red cells, and pH levels were similar to those found after storage in DEHP-plasticized containers under the same conditions. After 42 days, hemolysis levels were 0.56 +/- 0.21 percent and 0.42 +/- 0.17 percent in two series of units mixed weekly and 0.70 +/- 0.27 percent in units stored unmixed. Although even higher levels of hemolysis were observed in the units stored unmixed and used for 24-hour posttransfusion survival, the autologous red cell recovery results were excellent (83.2 +/- 5.1%, n = 8). BTHC-plasticized PVC is found to be a suitable material for 42-day storage of red cells in SAGM solution.

Incidence of legal abortion in Sweden after the Chernobyl accident

The number of legal abortions in Sweden increased around the time of the Chernobyl accident, particularly in the summer and autumn of 1986. Although there was no recording of reasons for legal abortions, one might have suspected this increase to be a result of fear and anxiety after the accident. However, seen over a longer time perspective, the increase in the number of abortions started before and continued far beyond the time of the accident. There was also a simultaneous and pronounced increase in the number of births during the years subsequent to the accident. Therefore, it seems unlikely that fear of the consequences of radioactive fall-out after the Chernobyl accident resulted in any substantial increase of the number of legal abortions in Sweden.

Surveillance of smoking during pregnancy in Sweden, 1983-1987

Since 1982, prospective information on smoking during early pregnancy is reported to the Swedish Medical Birth Registry for nearly all (93%) women who give birth. The present paper studies the validity of this information: effects on birthweight and perinatal mortality are very similar to those described previously in the literature. For each parity class, smoking decreases in inverse proportion to increasing maternal age; for each maternal age class, smoking increases with parity. A slight reduction in smoking rate is observed between 1983 and 1987, most pronounced for young women. There are marked geographic and social differences in the rate of smoking during pregnancy. This dataset can be used in the future to monitor the prevalence of smoking, and to study various factors associated with smoking and the impact of countermeasures taken against smoking during pregnancy. It can also be used to study possible associations between maternal smoking and rare events like congenital malformations and child cancer.

Clinical results in patients provided with conical crown retained dentures

During 1985 and 1986, 25 patients were treated with conical crown retained dentures, 26 in the maxillary arch and four in the mandible. The primary indication for having used these dentures was the presence of only a few teeth, mostly in unfavorable positions. Clinical findings after 24 to 43 months are reported and considered promising. Patients were generally satisfied and only comparatively small tissue changes were found.

Socio-economic variables and pregnancy outcome. 2. Infant and child survival

The effects of various social indicators on infant and child mortality were studied in Sweden with the use of a medical birth register to which census information was linked. Two years were studied: 1976 births linked to the 1975 census, and 1981 births linked to the 1980 census. Survival was followed to the age of 5 by linkage of the birth register with the death certificate register. The only statistically significant effect of a single socio-economic variable was that of housing conditions on perinatal death rate and postperinatal death rate up to the age of one. The family situation (e.g., cohabitation or not) had some effect, although it was not statistically significant. On the basis of cohabitation status and other social indicators, including housing conditions, we selected two groups: one privileged and the other underprivileged. Using crude mortality rates, we found no definite difference. There was evidence that the mortality rate had decreased more between 1976 and 1981 in the privileged than in the underprivileged group, but the difference may have been coincidental. After standardization for maternal age and parity, however, a difference appeared with a ratio of 1.14 between the underprivileged and the privileged groups, which was valid for deaths up to the age of one. After that age, no difference was seen. Following standardization for birthweight, the opposite was found: a higher weight-specific mortality rate in the privileged group than in the underprivileged group. The interpretation of these findings is discussed.

A quality study of a medical birth registry

A quality control study was made of the Swedish Medical Birth Registry. This registry used one mode of data collection during 1973-1981 and another from 1982 onwards. The number of errors in the register was checked by comparing register information with a sample of the original medical records, and the variability in the use of diagnoses between hospitals was studied. Different types of errors were identified and quantified and the efficiency of the two methods of data collection evaluated.

Lack of success with a combination of alanine and phosphoenolpyruvate as an additive for liquid storage of red cells at 4 degrees C

Red cells were stored at 4 degrees C in a storage solution containing alanine or alanine plus phosphoenolpyruvate (PEP). The intention was to investigate whether alanine and PEP might act synergistically to maintain a normal level of both red cell ATP and 2.3 diphosphoglycerate (2.3 DPG) under normal liquid storage conditions. Storage in the presence of alanine kept the red cell concentration of 2.3 DPG higher than the reference solution for an extended period of time, provided the initial pH was about 7.0 (37 degrees C). When the pH of the storage solution containing alanine plus PEP was lowered to facilitate the transport of PEP into the red cells, the concentration of 2.3 DPG was lowered to a rate equal to that in the reference solution. The level of ATP was also about the same as in the reference solution. The majority of the added PEP was continuously converted to 2 phosphoglycerate and 3 phosphoglycerate in the extracellular fluid. A small amount of unconverted PEP penetrated the red cell membrane when the pH went below 6.5; this occurred after 3 weeks of storage. The intracellularly located PEP, however, was not metabolized to 2.3 DPG to any significant extent within the first 6 weeks of storage. These findings indicate that PEP is not suitable as an additive for liquid storage of red cells at 4 degrees C. The combination of alanine and PEP that theoretically could be a suitable additive for liquid storage of red cells was not satisfactory in practice.

Socio-economic variables and pregnancy outcome. Birthweight in singletons

The effect of various social indicators on birthweight in singletons was studied in Sweden with the use of a medical birth registry to which census information was linked. Two years were studied: 1976 births linked to the 1975 census, and 1981 births linked to the 1980 census. The strongest social indicator affecting birthweight was family situation: whether the woman cohabited or not. After standardization for this variable, effects were still seen for occupation and type of housing. They were not secondary to maternal age or parity. By using cohabitation status, enhanced with other social indicators, two groups were selected: one privileged and one underprivileged. Distributions of birthweight and pregnancy duration in the two groups were compared with those formed in women being neither privileged nor under privileged and also working outside the home (because occupation entered the definitions of the two former groups). There were marked differences: underprivileged women had shorter pregnancy duration and, at term, their infants weighed slightly less than those of privileged women. These differences remain after standardization for maternal age and parity.

Pregnancy outcome in women working as dentists, dental assistants or dental technicians

Pregnancy outcome in women with work in dentistry was studied using various central health registries. A total of 8157 infants born of dentists, dental assistants, or dental technicians in 1976 or 1982-1986 in Sweden were studied with respect to perinatal survival, low birthweight, and malformations and compared with all births. The only deviating finding was that of a significantly low perinatal death rate. Specifically, no increase in a risk for spina bifida was seen and the upper 95% confidence limit for the risk ratio was 2.1. A study was also made of hospitalized spontaneous abortions in women with these occupations in the years 1980-1981. No significant deviations from expected values were found. In a small study of only 78 such pregnancies in 1964-1965, no increase in spontaneous abortion rate was seen. Only one infant was malformed (anencephaly): both its parents worked as dental technicians. None of the mothers of 220 infants with a neural tube defect born in 1965-1967 in Sweden was a dentist. We find no indications that this occupation represents a significant reproduction hazard at the present time in Sweden.

Modelling of reference values for size at birth

Data from the Swedish Medical Birth Registration, 1977-1981 were used to apply methods of constructing reference standards for size at birth. Using clinical information a 'healthy' sub-population was extracted. The conditional distributions of birthweight (BW) and birthlength (BL) for each week of Gestational age, and the conditional distribution of birthweight given birthlength were modelled using truncated Normal distributions, after making use of Box-Cox power transformations. Spline functions were then used in conjunction with a multiplicative method to obtain appropriate percentage point curves. Examples of this analysis are given.

Deaths and tumors among workers grinding stainless steel

This study examined a cohort of 1,164 males who, during the period 1927-1981, had been employed for 3 months or more in an industry that produced objects from stainless steel. These workers were exposed to the dust of grinding materials, grinding agents, and stainless steel. From 1975 to 1980, measurements of the total dust in the workroom air have shown levels of about one mg/m3 (consisting of chromium at about 0.1 mg/m3 and nickel at about 0.05 mg/m3) during grinding and several mg/m3 (chromium at about 0.01 mg/m3 and nickel at about 0.005 mg/m3) during polishing. Before 1950, the concentrations were probably considerably higher. Compared to a local reference population, a decrease in mortality (63 observed deaths, standardized mortality (SMR) = 0.79, 95% confidence limits CL = 0.61, 1.01) took place during the 1951-1983 observation period among 318 subjects who had been employed for at least five years, allowing a latency period of at least 20 years. Mortality from cancer of the colon/rectum increased (observed deaths = 6, SMR = 2.47, CL = 0.97, 5.58). The mortality from nonmalignant pulmonary disease did not increase (SMR = 0.29, CL = 0.01, 1.81). During the observation period (1958-1983), there was no significant overall excess of tumors (observed cases = 33, standardized morbidity [SMR] = 1.09, CL = 0.76, 1.54). However, tumor morbidity of the colon/rectum (observed 11, SMR = 2.83, CL = 1.47, 5.19) significantly increased. There were no cases of respiratory cancer (expected 4.7, CL = 0, 0.21). Thus, the results indicate an increased risk of intestinal cancer.(ABSTRACT TRUNCATED AT 250 WORDS)

Purine metabolism in normal and ITP-pyrophosphohydrolase-deficient human erythrocytes

Fresh and stored erythrocytes from normal and ITP-pyrophosphohydrolase (ITP-ase, EC 3.6.1.19) deficient individuals were incubated with hypoxanthine, guanine, allopurinol, and inosine. Differences in the purine metabolism between the normal and the ITP-ase deficient erythrocytes were observed only in the IMP-ITP cycle. Hypoxanthine, guanine and allopurinol were converted to nucleotides at the same rate. Hypoxanthine (2.5 mumol/l) inhibited the salvage of allopurinol (40 mumol/l). A slow decrease (0.7%/day) in salvage rate was observed in both types of cells upon storage at +4 degrees C. Erythrocyte ITP-ase activity was measured in a reference sample group of 48 healthy volunteers. Two distinct groups were found with mean activities equal to 48.3 +/- 13.1 nkat/g Hb (means +/- SD, n = 38) and 11.4 +/- 4.3 nkat/g Hb (n = 10). In two previously selected subjects, the ITP-ase activity was 0.2 and 2.4 nkat/g Hb. A hypothetical genetic mechanism is discussed. The maximal energy turnover in the IMP-ITP cycle during hypoxanthine incubation was found to be less than 10% of the basal erythrocyte energy turnover.

Studies on the mechanism of human red cell loss of viability during storage at +4 degrees C in vitro. III. Effects of mixing during storage

Red cells supended in saline-adenine-glucose-mannitol solution were stored for 42 days at +4 degrees C. One portion was packed by centrifugation and stored unmixed; the other portion was mixed once a week. Red cell fluidity and adenine nucleotide concentration were significantly lower in unmixed than in mixed units and also differed within the packed layer, showing a decrease towards the bottom. Hemolysis was 2.5-fold and microvesiculation 5-fold higher in cells stored unmixed. It is suggested that, during liquid storage, an early accumulation of acid metabolites in the bottom part of packed red cells may play an important role both for adenylate loss and microvesiculation, but lack of membrane-stabilizing action of diethylhexylphthalate may in part explain the latter. Both of these two changes are factors associated with impaired red cell viability. Mixing appears essential to optimize storage conditions for red cells.

Maternal occupation and delivery outcome: a study using central registry data

The outcome of all deliveries in Sweden in 1981 has been studied. The occupation of each mother was obtained by record linkage using census data from November 1980. Rates of perinatal deaths or congenital malformations did not vary according to maternal occupation, more than might be expected to happen by chance, which indicated that maternal occupation is not a major factor in the outcome of these deliveries. There was variation in the rates of low birth weight infants, according to maternal occupation, which may have been an effect of socioeconomic factors associated with the occupation. Within occupational groups, working situations may exist which entail an increased teratogenic risk, but in order to reveal such risks, studies with a different design must be carried out.

Birth weight distribution as an indicator of environmental effects on fetal development

A medical birth registry was used for a geographical analysis of birth weight distribution. Nearly 900,000 Swedish singleton births, 1973-1981, were used for an analysis of the effect of some variables and for standardization for these variables. A marked change in the rate of low birth weight infants (LBW, less than 2,500 g) was seen in the country between 1976 and 1977. A U-formed effect of maternal age and of parity was demonstrated. A marked interaction between the effects of these two variables existed. Two social groups were compared and the well-known high rate of LBW infants associated with low socioeconomic conditions was demonstrated. Standardization for the variables mentioned influenced this effect only little but reduced the difference between the social groups concerning infants above 3.5 kg weight. The background data were used for analysis of restricted geographical areas.