Month of birth and risk of developing insulin dependent diabetes in south east Sweden

Lack of association between month of birth and risk of developing type 1 diabetes in Brazil: a 40-year analysis

OBJECTIVES

Seasonal environment at birth may influence diabetes incidence in later life. We sought evidence for this effect and analyzed the association between the month of birth and the risk of developing type 1 diabetes mellitus (T1DM).

METHODS

This was a cohort study carried out with 814 patients diagnosed with T1DM in the region of Bauru - São Paulo State, Brazil, receiving medical care in a private Endocrinology clinic or in the public Brazilian National Health Care System, from 1981 to 2021. All live births that occurred in São Paulo State between 1974 and 2020 were classified by month of birth and were considered as the control group.

RESULTS

We found no statistically significant difference (χ2=16.31, critical 19.68) between the month of birth and risk of developing T1DM, when comparing our patients with the background population of the region. There was no association between the month of birth, sex, age at diagnosis, duration of symptoms before diagnosis, self-reported color, and socioeconomic status.

CONCLUSIONS

We found no association between month of birth and the risk of developing T1DM in this highly admixed South American population. Our data suggest that our population heterogeneity and geographic location may be important factors in the development of T1DM. Future prospective studies, evaluating environmental factors that may confer risk or protection to the disease, are warranted.

Hypothesis: Viral infections of pregnant women may be early triggers of childhood type 1 diabetes and other autoimmune disease

Children and adolescents with early onset autoimmune diseases have a different seasonality of month of birth than the general population. This pattern is consistent with an infection during pregnancy affecting the fetus or an infection immediately after birth that act as early triggers of the autoimmune diseases. We present data supporting the use of Rotavirus vaccinations in the reduction of incidence of childhood T1D and propose further investigations into whether other anti-virus vaccinations may reduce the burden of other autoimmune diseases such as multiple sclerosis, atopic dermatitis, psoriasis and subtypes of rheumatoid arthritis, Hashimoto thyroiditis.

Month of birth and the risk of developing type 1 diabetes among children in the Swedish national Better Diabetes Diagnosis Study

AIM

Previous studies have reported an association between month of birth and incidence of type 1 diabetes. Using population-based data, including almost all newly diagnosed children with type 1 diabetes in Sweden, we tested whether month of birth influences the risk of type 1 diabetes.

METHODS

For 8761 children diagnosed with type 1 diabetes between May 2005 and December 2016 in the Better Diabetes Diagnosis study, month of birth, sex and age were compared. Human leucocyte antigen (HLA) genotype and autoantibodies at diagnosis were analysed for a subset of the cohort (n = 3647). Comparisons with the general population used data from Statistics Sweden.

RESULTS

We found no association between month of birth or season and the incidence of type 1 diabetes in the cohort as a whole. However, boys diagnosed before 5 years were more often born in May (p = 0.004). We found no correlation between month of birth and HLA or antibodies.

CONCLUSION

In this large nationwide study, the impact of month of birth on type 1 diabetes diagnosis was weak, except for boys diagnosed before 5 years of age, who were more likely born in May. This may suggest different triggers for different subgroups of patients with type 1 diabetes.

Month of birth is associated with the subsequent diagnosis of autoimmune hypothyroidism. A nationwide Danish register-based study

BACKGROUND

The triggering of thyroid autoimmunity in the genetically susceptible remains a conundrum. Environmental exposures during gestation and/or early postnatally have proponents, as suggested in diabetes mellitus, with a higher incidence of births during spring and summer. Whether the development of autoimmune hypothyroidism (AIT) is influenced by month or season of birth is less clear.

METHOD

Nationwide cohort study of 111 565 individuals diagnosed with AIT and four euthyroid controls per case, matched according to age and sex, were identified from Danish health registers. Differences in month of birth across the year were evaluated by the Walter-Elwood test. The risk of patients with AIT being born in a certain month or season of the year was calculated using a Cox regression model.

RESULTS

There was a significant difference in birth month between cases and controls, P<.001. Individuals with AIT had a significantly increased risk of being born in June (Hazard ratio 1.04; 95% Confidence interval (CI): 1.02-1.08) and in the summer (June-August; HR 1.02; 95%CI: 1.01-1.04).

CONCLUSION

In this large-scale nationwide cohort study, we found a higher risk of AIT when born in the summer season or more specifically in June, supporting the hypothesis that seasonal variations in exposures-gestationally and/or early postnatally-may contribute to the development of AIT.

Are low sun exposure and/or vitamin D risk factors for type 1 diabetes?

Ultraviolet radiation and vitamin D, with their known immunosuppressive effects, have the potential to delay or inhibit type 1 diabetes.

Exposure to sunshine early in life prevented development of type 1 diabetes in Danish boys

BACKGROUND

We aimed to assess the association between exposure to sunshine during gestation and the risk of type 1 diabetes (T1D) in Danish children.

METHODS

The study population included 331,623 individuals born in Denmark from 1983 to 1988; 886 (0.26%) developed T1D by the age of 15 years. The values of sunshine hours were obtained from the Danish Meteorological Institute. Gestational exposure to sunshine was calculated by summing recorded monthly sunshine hours during the full 9 months prior to the month of birth. The linear variable then was split into two categories separated by the median value.

RESULTS AND CONCLUSIONS

Cox regression models showed that more sunshine during the third gestational trimester was associated with lower hazards (HR) of T1D at age 5-9 years in males: HR (95% CI): 0.60 (0.43-0.84), p=0.003. Our results should be considered in the context of evidence-based recommendations to the public about skin protection from the sun.

Perinatal risk factors increase the risk of being affected by both type 1 diabetes and coeliac disease

AIM

This study investigated whether perinatal factors influenced the risk of a double diagnosis of type 1 diabetes and coeliac disease.

METHODS

We used multinomial logistic regression models to study the associations between perinatal factors, gender, mode of delivery, season of birth and the risk of type 1 diabetes, coeliac disease or both, in Swedish-born singleton children.

RESULTS

We found that 4327 of the 768 395 children (0.6%) had been diagnosed with type 1 diabetes, 3817 (0.5%) had been diagnosed with coeliac disease and 191 (0.02%) were affected by both diseases. If the children already had type 1 diabetes (n = 4518), the absolute risk of being affected by coeliac disease increased to 4.2% (n = 191). Children with both type 1 diabetes and coeliac disease were more likely to be female (OR = 1.48, 95% CI = 1.01-1.97), delivered by Caesarean section (OR = 1.60, 95% CI = 1.07-2.39), have native-born Swedish mothers (OR = 4.84, 95% CI = 1.96-11.97) or be born during the summer months (OR = 1.43, 95% CI = 1.07-1.92).

CONCLUSION

The increased risk of being affected by a double diagnosis of type 1 diabetes and coeliac disease was modulated by perinatal risk factors. This suggests that early life events are important when it comes to children with type 1 diabetes also developing coeliac disease.

The role of vitamin d deficiency in the incidence, progression, and complications of type 1 diabetes mellitus

The "nonclassic" role of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has been recently widely recognized. In type 1 diabetes mellitus (T1D), it plays an immunomodulatory role through the vitamin D receptor (VDR) present on pancreatic and immune cells. Specific VDR allelic variants have been associated with T1D in many countries. Furthermore, vitamin D deficiency has been prevalent in T1D, and the seasonal and latitude variability in the incidence of T1D can be partly explained by the related variability in vitamin D level. In fact, retrospective studies of vitamin D supplementation during pregnancy or infancy showed a lower incidence of T1D. We will review the different mechanisms of the vitamin D protective effect against insulitis and present the available data on the role of vitamin D deficiency in the control, progression, and complications of T1D.

Population density determines the direction of the association between ambient ultraviolet radiation and type 1 diabetes incidence

BACKGROUND

Type 1 diabetes incidence has increased rapidly over the last 20 years, and ecological studies show inverse latitudinal gradients for both incidence and prevalence. Some studies have found season of birth or season of diagnosis effects. Together these findings suggest an important role for environmental factors in disease etiology.

OBJECTIVE

To examine whether type 1 diabetes incidence varies in relation to ambient ultraviolet radiation (UVR) in Australian children.

METHODS

We used case records of 4773 children aged 0-14 yr from the Australian National Diabetes Register to estimate type 1 diabetes incidence in relation to residential ambient UVR, both as a continuous variable and in four categories. We examined season of birth and season of diagnosis and variation in these parameters and in age at diagnosis, in relation to ambient UVR.

RESULTS

Overall incidence was 20 per 100 000 population with no sex difference. There was a statistically significant trend toward winter diagnosis (adjusted RR = 1.22, 95% CI 1.13-1.33, p<0.001) but no apparent season of birth effect. Incidence in the highest UVR category was significantly lower than in the lowest UVR category (RR = 0.85, 95% CI 0.75-0.96). We found an inverse association between incidence and ambient UVR that was present only at low population densities; at high population densities type 1 diabetes incidence increased with increasing ambient UVR.

CONCLUSION

In low population density, largely rural environments, ambient UVR may better reflect the personal UV dose, with the latter being protective for the development of type 1 diabetes. This effect is lost or reversed in high population density, largely urban, environments.

Epidemiology of type 1 diabetes

This article describes the epidemiology of type 1 diabetes mellitus (T1D) around the world and across the lifespan. Epidemiologic patterns of T1D by demographic, geographic, biologic, cultural, and other factors in populations are presented to gain insight about the causes, natural history, risks, and complications of T1D. Data from large epidemiologic studies worldwide indicate that the incidence of T1D has been increasing by 2% to 5% worldwide and that the prevalence of T1D is approximately 1 in 300 in the United States by 18 years of age. Research on risk factors for T1D is an active area of research to identify genetic and environmental triggers that could potentially be targeted for intervention. Although significant advances have been made in the clinical care of T1D with resultant improvements in quality of life and clinical outcomes, much more needs to be done to improve care of, and ultimately find a cure for, T1D. Epidemiologic studies have an important ongoing role to investigate the complex causes, clinical care, prevention, and cure of T1D.

The effect of solar cycles on human lifespan in the 50 United states: variation in light affects the human genome

This paper describes the Sun's effect on the human genome as it relates to lifespan and expands our previous study from the State of Maine to the entire United States and the District of Columbia. In the current study we report that those persons conceived and likely born during the peaks (MAX approximately 3years) of approximately 11-year solar cycles lived an average 1.7years less than those conceived and likely born during non-peaks (MIN approximately 8years). Increased energy at solar MAX, albeit relatively a small 0.1% increase from MIN, apparently modifies the human genome/epigenome and engenders changes that predispose to various diseases, thereby shortening lifespan. It is likely that same energy increases beneficial variety in the genome which may enhance adaptability in a changing environment. This study also reports that living at higher elevations increases exposure to ultraviolet radiation (UVR) and increases the difference between MAX and MIN in the six states at the highest elevations of their population centroids by approximately 13%, further shortening average lifespan about 3 months. How solar energy affects the genome is still not clear. The mechanism could be quantum mechanical (direct effects at a distance) similar to photosynthesis, or mediated by maternal hormones, chemokines or cytokines. The hypothesis is that specific wavelengths of UVR, experienced at critical times in development as at conception or early gestation, and with specific intensity or rate of change, modulates the expression of human diseases. This hypothesis could be readily testable in mice bred to manifest specific diseases.

Vitamin D status during pregnancy and aspects of offspring health

Low maternal vitamin D levels during pregnancy have been linked to various health outcomes in the offspring, ranging from periconceptional effects to diseases of adult onset. Maternal and infant cord 25(OH)D levels are highly correlated. Here, we review the available evidence for these adverse health effects. Most of the evidence has arisen from observational epidemiological studies, but randomized controlled trials are now underway. The evidence to date supports that women should be monitored and treated for vitamin D deficiency during pregnancy but optimal and upper limit serum 25(OH)D levels during pregnancy are not known.

Association of type 1 diabetes with month of birth among U.S. youth: The SEARCH for Diabetes in Youth Study

OBJECTIVE

Seasonal environment at birth may influence diabetes incidence in later life. We sought evidence for this effect in a large sample of diabetic youth residing in the U.S.

RESEARCH DESIGN AND METHODS

We compared the distribution of birth months within the SEARCH for Diabetes in Youth Study (SEARCH study) with the monthly distributions in U.S. births tabulated by race for years 1982-2005. SEARCH study participants (9,737 youth with type 1 diabetes and 1,749 with type 2 diabetes) were identified by six collaborating U.S. centers.

RESULTS

Among type 1 diabetic youth, the percentage of observed to expected births differed across the months (P = 0.0092; decreased in October-February and increased in March-July). Their smoothed birth-month estimates demonstrated a deficit in November-February births and an excess in April-July births (smoothed May versus January relative risk [RR] = 1.06 [95% CI 1.02-1.11]). Stratifications by sex or by three racial groups showed similar patterns relating type 1 diabetes to month of birth. Stratification by geographic regions showed a peak-to-nadir RR of 1.10 [1.04-1.16] in study regions from the northern latitudes (Colorado, western Washington State, and southern Ohio) but no birth-month effect (P > 0.9) in study regions from more southern locations. Among type 2 diabetic youth, associations with birth month were inconclusive.

CONCLUSIONS

Spring births were associated with increased likelihood of type 1 diabetes but possibly not in all U.S. regions. Causal mechanisms may involve factors dependent on geographic latitude such as solar irradiance, but it is unknown whether they influence prenatal or early postnatal development.

Before-birth climatologic data may play a role in the development of allergies in infants

While an exacerbation in allergic symptoms corresponding to seasons has long been reported, few studies have investigated the association between the season of birth and allergic disorders. The aim of this study was to investigate whether the climatologic data before and after birth affected the incidence of atopic dermatitis (AD) and the results of allergy-related blood tests in early infancy. From February 1995 to January 2000, 2136 infants were tested for AD and followed for 12 months. AD patients were tested by using allergy-related blood tests. Data were compared according to the month of birth and the climatologic data using a computed statistical software package. Six hundred and thirty infants had AD before 12 months old, and significant differences were found according to the season of birth (p < 0.0001). Infants born in spring showed the lowest (22.3%) incidence, while those born in autumn showed the highest (34.6%). In 369 patients, total serum IgE levels, and serum specific IgE levels with egg white at 3 months old were also different according to the season of birth. All of these levels were lower in patients born in spring and summer, and higher in patients born in autumn and winter. Furthermore, the cumulative sunshine amount during the 3 months before and after birth was inversely correlated, while the average temperature over the 3 months before birth was positively correlated to the incidence of AD according to the month of birth. The climatologic data around birth may play an important role in whether an infant develops allergies.

Seasonal variation in the diagnosis of type 1 diabetes in south-east Sweden

With the aim to survey the seasonal pattern of diagnosis of type 1 diabetes we included all 1903 children <16 years of age and who had been diagnosed with type 1 diabetes between 1977 and 2001 in the south-east of Sweden. To investigate the seasonal pattern a mixture of two cosine functions was included in a logistic regression model. There was a clear seasonal variation over the years (p<0.001). Children in the oldest age group (11-15 years) showed the most obvious seasonal variation (p<0.001). Children with a short duration of symptoms had about the same seasonal variation as children with a long duration. Both children with and without an infection 3 months prior to diagnosis showed significant seasonal variation (p<0.001) although the seasonal pattern differed between the two groups (p<0.001). As the incidence of diabetes increased during the 25 years the study period was divided into periods of 5 years and it was only during the two last periods that significant seasonal variation occurred. There is a clear seasonal variation in diagnosis of type 1 diagnosis in children and the results suggest that children with a less aggressive disease process at diagnosis were most responsible for this variation. Children with and without prior infection showed a different seasonal pattern.

Seasonal variation of nutrient intake in pregnancy: effects on infant measures and possible influence on diseases related to season of birth

OBJECTIVES

To determine, firstly, if there was any seasonal effect on nutrient intake during pregnancy and birth measures, secondly, if there was any relationship between maternal nutrient intake and infant birth measures according to season and thirdly, to consider the hypothesis that seasonal change in nutrient intake during pregnancy might affect health in later life of some women's offspring.

DESIGN

Pilot study to determine number of days required to characterize group intake followed by a prospective cohort study.

SETTING

Patients attending a city ante-natal clinic in the lower North Island, New Zealand.

SUBJECTS

A total of 214 healthy mostly European pregnant women volunteers, entering the second trimester of pregnancy, of whom 10 miscarried and seven withdrew.

METHODS

Subjects were visited in months 4 and 7 of pregnancy, and months 2, 6 and 12 after birth. Height, weight and skinfolds were measured and questionnaires to determine personal details administered at these times. Subjects recorded 8 days of weighed diets in both the fourth and seventh month. Health records were used to supply infant measures.

RESULTS

Significant (P</=0.05) seasonal variations in fat, carbohydrate, vitamin C and D, B vitamins, beta-carotene, sodium, potassium, calcium, phosphorous, sulfur, sodium, chloride, zinc and selenium intakes were found. No significant 'main effect' difference in gestational age, infant birthweight, and head circumference was found with season. However, there were significant interactions (P</=0.05) for each birth season between birth measures and specific maternal nutrients at months 4 and 7 of pregnancy.

CONCLUSION

The significant seasonal variations in nutrient intake in pregnant women, and significant influence of nutrient intake on birth measures in different seasons, suggests seasonal nutrient variation may also affect fetal development at a cellular level. This supports our hypothesis that the development of conditions related to season of birth, including schizophrenia, multiple sclerosis, type I diabetes and longevity, may be influenced by seasonal variation in nutrient intake during pregnancy.

Seasonality of birth in children and young adults (0-29 years) with type 1 diabetes in Ukraine

AIMS/HYPOTHESIS

Numerous epidemiological studies have shown differences in seasonality of birth patterns between the general population and the group who develop type 1 diabetes mellitus. This finding indicates that environmental factors operating during pre- and/or postnatal development could be aetiologically important. We examined whether the pattern of month of birth for type 1 diabetes patients in Ukraine differs from that for total live births.

METHODS

Data consist of prevalent cases of type 1 diabetes in Ukraine by the end of 2003. Cases are restricted to persons born after 1 January 1960, diagnosed with type 1 diabetes before the age of 30 years (n = 20,117). People born during the same time in the general population (n = 29,105,560) were the reference standard. Seasonal patterns were estimated using logistic regression with harmonic terms.

RESULTS

We found a strongly significant seasonal pattern of type 1 diabetes incidence rates (p < 0.001), with the lowest rates in December and the highest in April. The rate ratio between the extremes was 1.32 (95% CI 1.27-1.39). Tests for seasonal patterns in subgroups defined by sex and age or by sex and date of birth were all significant with p values less than 0.02. We found no interactions with sex (p = 0.142) or age at diagnosis (p = 0.207), but found a strong interaction with period of birth (p < 0.0001).

CONCLUSIONS/INTERPRETATION

The results obtained indicate that early-life factors linked to seasons may influence type 1 diabetes risk later in life.

UVR, vitamin D and three autoimmune diseases--multiple sclerosis, type 1 diabetes, rheumatoid arthritis

We review the evidence indicating a possible beneficial role for UVR on three Th1-mediated autoimmune diseases: multiple sclerosis, type 1 diabetes and rheumatoid arthritis in relation to recent developments in photoimmunology. Recent work suggests that UVR exposure may be one factor that can attenuate the autoimmune activity leading to these three diseases through several pathways involving UVB and UVA irradiation, UVR-derived vitamin D synthesis and other routes such as alpha-melanocyte-stimulating hormone, calcitonin gene related peptide and melatonin. Ecological features, particularly a gradient of increasing prevalence of multiple sclerosis and type 1 diabetes with higher latitude, provide some support for a beneficial role of UVR. Analytical studies provide additional support, particularly as low vitamin D has been prospectively associated with disease onset for all three diseases, but are not definitive. Randomized controlled trial data are required. Further, we discuss how associated genetic studies may assist the accumulation of evidence with regard to the possible causal role of low UVR exposure and/or low vitamin D status in the development of these diseases.

Clinical characteristics at onset of Type 1 diabetes in children diagnosed between 1977 and 2001 in the south-east region of Sweden

To survey clinical characteristics at diagnosis for children diagnosed with Type 1 diabetes during 25 years in the south-east part of Sweden we included all 1903 children < 16 years of age and who had been diagnosed between 1977 and 2001 in the south-east region of Sweden. A nurse or doctor in the diabetes team obtained information from medical records. Over the 25 years the mean duration of symptoms prior to diagnosis was 17.8 +/- 26.4 days and the mean glucose level at diagnosis was 23.6 +/- 9.7 mmol/l. Three percent of the children (n = 50) had a pH value < or = 7.1. The youngest children (0-5 years) had shorter duration of symptoms, lower blood-glucose levels and less often had ketonuria than the oldest children (11-15 years) but more often suffered from infections prior to diagnosis. The proportion of children diagnosed in the group 0-5 years of age increased over the study-period, apart from the last 5 years, while children with pH value < or = 7.3 decreased significantly as did the proportion of children with ketonuria or infection. The clinical characteristics at diagnosis of diabetes are heterogeneous, especially in the oldest age group. Some characteristics varied with time.

Geographical mapping of type 1 diabetes in children and adolescents in south east Sweden

STUDY OBJECTIVE

As earlier studies have shown space-time clusters at onset of type 1 diabetes in the south east region of Sweden we investigated if there also has been any geographical clusters of diabetes in this region.

DESIGN

The place of residence (coordinates) at the time of diagnosis were geocoded in a geographical information system (GIS). All children diagnosed with type 1 diabetes up to 16 years of age at diagnosis between 1977-1995 were included. The population at risk was obtained directly from the population registry for the respective years and geographical area levels.

SETTING

South east region of Sweden containing 5 counties, 49 municipalities, and 525 parishes.

MAIN RESULTS

A significant geographical variation in incidence rate were found between the municipalities (p<0.001) but not between the counties. The variation became somewhat weaker when excluding the six largest municipalities (p<0.02). In municipalities with increased risk (>35.1/100 000) the major contribution comes from children in age group 6-10 years of age at diagnosis. There were no obvious differences between the age groups in municipalities with decreased risk (<20.1/100 000). Boys and girls had about the same degree of geographical variation.

CONCLUSIONS

Apart from chance, the most probable explanation for the geographical variation in the risk for children and adolescents to develop type 1 diabetes between the municipalities in the region is that local environmental factors play a part in the process leading to the disease.

Seasonality of month of birth among African American children with diabetes mellitus in the city of Chicago

AIM

To study the seasonality of month of birth among African American children with insulin-treated diabetes mellitus (DM) in the city of Chicago, in order to determine whether perinatal exposures play a significant role in diabetes risk among children of non-European origin.

METHOD

The Chicago Childhood Diabetes Registry ascertains new cases of insulin-treated DM among minority children < 18 years of age; these cases were compared with birth certificate data for the general African American population in Chicago. The chi2 test and Poisson regression were used to compare the pattern of month of birth of children with DM (n = 604) to that of the general population (n = 758,658) over the same period of years (1968-1995).

RESULTS

In a month-by-month comparison, there were significantly fewer children who later developed DM born during October (chi2 = 6.74, df = 1). This seasonal pattern was stronger among males (n = 284) than females (n = 320), and among those who apparently developed type 2 DM (n = 155) compared to those who developed type 1 DM (n = 449). Children who were diagnosed between 15 and 17 years of age (n = 131) demonstrated significant seasonality (chi2 = 27.6, df = 11) compared to the general population.

CONCLUSIONS

The apparent protective effect of October birth, and the significant overall seasonality among those diagnosed at ages 15-17 years, suggest the possibility that seasonal environmental factors at conception, during pregnancy or in the neonatal period may affect DM risk in adolescence. The greater impact of month of birth in adolescent type 2 DM patients is surprising and seems to indicate a role for mechanisms other than the immunological ones previously suggested.

Ultraviolet radiation and autoimmune disease: insights from epidemiological research

This review examines the epidemiological evidence that suggests ultraviolet radiation (UVR) may play a protective role in three autoimmune diseases: multiple sclerosis, insulin-dependent diabetes mellitus and rheumatoid arthritis. To date, most of the information has accumulated from population studies that have studied the relationship between geography or climate and autoimmune disease prevalence. An interesting gradient of increasing prevalence with increasing latitude has been observed for at least two of the three diseases. This is most evident for multiple sclerosis, but a similar gradient has been shown for insulin-dependent diabetes mellitus in Europe and North America. Seasonal influences on both disease incidence and clinical course and, more recently, analytical studies at the individual level have provided further support for a possible protective role for UVR in some of these diseases but the data are not conclusive. Organ-specific autoimmune diseases involve Th1 cell-mediated immune processes. Recent work in photoimmunology has shown ultraviolet B (UVB) can specifically attenuate these processes through several mechanisms which we discuss. In particular, the possible contribution of an UVR-induced increase in serum vitamin D (1,25(OH)2D3) levels in the beneficial immunomodulation of these diseases is discussed.

Lifespan depends on month of birth

Month of birth influences adult life expectancy at ages 50+. Why? In two countries of the Northern Hemisphere-Austria and Denmark-people born in autumn (October-December) live longer than those born in spring (April-June). Data for Australia show that, in the Southern Hemisphere, the pattern is shifted by half a year. The lifespan pattern of British immigrants to Australia is similar to that of Austrians and Danes and significantly different from that of Australians. These findings are based on population data with more than a million observations and little or no selectivity. The differences in lifespan are independent of the seasonal distribution of deaths and the social differences in the seasonal distribution of births. In the Northern Hemisphere, the excess mortality in the first year of life of infants born in spring does not support the explanation of selective infant survival. Instead, remaining life expectancy at age 50 appears to depend on factors that arise in utero or early in infancy and that increase susceptibility to diseases later in life. This result is consistent with the finding that, at the turn of the last century, infants born in autumn had higher birth weights than those born in other seasons. Furthermore, differences in adult lifespan by month of birth decrease over time and are significantly smaller in more recent cohorts, which benefited from substantial improvements in maternal and infant health.