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Are you a summer, spring, autumn, or winter baby? To what extent do you feel when you were born affects you? I’m not just talking about what type of celebrations you are able to have or your horoscope. Could there be a connection between what time of year you were born and your risk for developing certain chronic diseases?

In the past few years, more and more research has investigated whether there is any kind of connection between when you are born (the season) and susceptibility to certain chronic conditions and even mortality risk. Let’s see what the literature has to say about the validity of any of these connections.

There are some things you cannot change about yourself, such as your genetics and at what time of year you were born. However, discovering important connections with certain potential risk factors for chronic health conditions may provide insights into areas you may be more predisposed to be affected by. Then, you can take a look at the things you CAN control, such as your diet and lifestyle, to modify the risk factor as much as possible. As you read the summaries below, please realize the studies focused on specific populations, and the results may not apply directly to you or your family. I encourage you to discuss any of your health-related questions with your healthcare practitioner.

Why Season of Birth Matters

There are many factors at play that may influence a connection between health, disease risk, and season of birth, many of which are environmental characteristics, such as early sunlight exposure, prenatal vitamin D synthesis and status, temperature, humidity, and time spent outdoors. There are also seasonal changes that affect nutrient status, such as fresh fruit and vegetable availability. Infection risk and other prenatal stress may also change from season to season, impacting the early health of a child and subsequent development and/or later health. How exactly do these factors impact health? Many of the answers still elude researchers, but there are a few hypotheses and studies pointing to potential mechanisms.

DNA methylation, which turns on or off certain genes, may play some role. One cohort study in the Isle of Wight looking at the association between season of birth and allergic diseases found an association between season of birth and eczema. Furthermore, they looked at DNA methylation and found associations between methylation of certain genes and allergic disease, detecting 92 CpGs associated with allergic disease that were also significantly correlated with season of birth.

Many researchers point to sun exposure and vitamin D status as a potential factor for health, especially with the role of vitamin D. One study in Sweden looked at the link between season of birth, neonatal vitamin D status, and later in life health outcomes. They found that those born after the winter had lower neonatal vitamin D compared to those born after the summer (31.5 nmol/L compared to 48.5 nmol/L).

There may be confounding factors beyond just when someone is born that are harder to account for in these studies. One interesting study found that there may also be a socioeconomic element at play. The researchers looked through census data to see if there were any differences in the family background of those born in different seasons. They found that women who gave birth in the winter tended to be less educated, were less likely to be married, and were younger, all of which could impact prenatal and childhood health, affecting health outcomes in later life as well. They found a 10% decrease in teenage births from January to May, which was a significant drop.

The researchers found the biggest differences in seasons were planned births, while unplanned births tended to stay relatively constant. This means that women who used contraception and other methods to plan when to have their child tended to stop contraception at a time that would increase the chance of having a child in better weather conditions, often the spring and summer months rather than during winter. Anticipated weather during birth played a larger role than weather at conception. When looking at birth numbers for women who did not plan on becoming pregnant, there was not the same seasonal fluctuation. Thus, when the total number of births was combined, the biggest different on seasonal changes of birth is derived from planned pregnancies for better weather.

Although the exact mechanisms remain unknown, many studies have found correlations between several chronic diseases and season of birth, as you will see. Likely, the cause is a multi-factorial process.

Allergy & Autoimmune Disorders

One area of health with several studies linking season of birth and disease risk relates to immune health, specifically the risk of developing allergic and autoimmune diseases.

Multiple sclerosis (MS) is widely researched, with many studies finding spring births at a higher risk for developing the disease. In one systematic review and meta-analysis, there were more patients with multiple sclerosis who had been born in the spring compared to autumn, with an odds ratio of 1.14. November had the fewest MS births, with an odds ratio of 0.96. A multivariate meta-analysis looking at 22 studies analyzing the number of MS births for each month found an increased risk for MS in those born in March, April, and May and lowest risk in November. Another systematic review consistently found more cases in the spring and less in the fall, although this effect was more prominent in areas that had low sunlight exposure compared to areas with high sunlight exposure. A study looking at serum MS metabolites found an increased level of metabolites in the spring and fall to ne a higher risk factor for MS than in other seasons.

The season of birth is not consistent for which disease someone may have a higher risk. For example, children born in autumn have an increased chance of developing celiac disease with an incidence ratio of 1.22. Lower levels of vitamin D in the diet and the presences of enteric and gastrointestinal viral pathogens may contribute to the relationship between celiac disease and season of birth. Another study in Italy supports this, finding that children born in the summer have a higher risk for celiac disease than those in other seasons.

Not all autoimmune disorders may have a link to season of birth. For example, one nationwide cohort study in Denmark with more than 56,000 participants found no association for season of birth and Graves’ disease.

Allergic disease also may be affected by when you were born. In one systematic review and meta-analysis, the researchers found significantly higher odds of having elevated IgE levels in cord blood in those who were born in winter, which could impact risk for allergic diseases. Compared to summer, winter births had an odds ratio of 1.24 for IgE levels above 0.1 IU/ml and a 1.30 odds ratio for IgE equal to or greater than 0.5 IU/ml, although it was not as significant. One study conducted in Poland using data from an Epidemiology of Allergic Diseases in Poland (ECAP) project using both a questionnaire-based study and clinical assessment found that participants born between May and July frequently has elevated IgE antibodies. Additional studies have found correlations with other allergic diseases.

A logistic regression model on 4323 participants of the Japan Environment and Children’s study looked at the relationship between season of birth and allergic rhinitis or pollinosis. The data demonstrated that spring and summer births were associated with the development of pollinosis. The relationship between exposure to allergens and vitamin D status has previously been linked to the development of pollinosis, but the results of this study conclude that weather conditions and the rate of respiratory infections at both should be analyzed further.

One pilot study found a higher frequency of autumn-winter births in children with early-onset food allergies. An analysis of Medicaid claims data from 2009-2010 found that children born in the fall were evidenced to be associated with a higher incidence of food allergies and atopic dermatitis. Further, 17 years of medical evaluations from Israel found that being born in the summer increased the risk of onset of asthma compared to those born in fall.

Behavioral and Mental Health

Another widely-researched area looking at links to season of birth and associated disease risk is mental and behavioral health.

One of the best studied is schizophrenia, with several studies demonstrating those born in the winter have a higher risk, especially those with early and mid-onset of the disease. However, some studies have challenged this concept, finding no correlation. For instance, one systematic meta-analysis found a small significant higher rate in patients diagnosed with schizophrenia who were born in winter, while patients born in summer had a small significant lower rate. However, these results only applied to births in the Northern hemisphere.

Another study looked at data from 98,457 patients and healthy controls in Brazil from the National Datacenter of Medical Promptuary and found that being born in August posed the greatest vulnerability for the development of manic depressive disorder, anxiety-related disorder, and schizophrenia. Examining 957 young adult patients under 25 years of age, researchers found that patients born in the spring had increased odds of early-onset bipolar affective disorder.

This increased risk may be due to genetics, specifically those involved in cytokines, combined with an increased risk of prenatal infection. A review on environmental risk factors of Schizophrenia and Bipolar Disorder concluded that there is an interaction between IL-4 ( an inflammatory cytokine) and demonstrated that seasonality, and the associated immune responses may be corrected with the development of these conditions.

The impact on emotional and behavioral health may begin early, with one study in Japan finding that there was a relationship between season of birth and emotional and behavioral regulation skills at 18 months old. Those born in the spring and summer had higher effortful control than those born in winter, and those born in the spring also had less aggression than those born in winter.

Obsessive compulsive disorder (OCD) may also be impacted by season of birth, especially in men. In one study from Taiwan looking at the birth month of patients diagnosed with OCD, there were fewer births from March to July and more births from August to November compared to the general population, with a relative risk of 0.85 and 1.19, respectively. When stratified, it remained present in men but not in women.

It is not just mental health disorders that may be linked to season of birth; behavioral and learning problems may also have an association. One longitudinal study found that those who were born in summer were more likely to have ADHD symptoms compared to those born in the spring. Learning disorders may also have a link to season of birth. Data from the UK’s Millennium Cohort Study found that summer and autumn births, among other predictors, were associated with a higher rate of developmental dyslexia.

Obesity and Metabolic Syndrome

Although results are mixed, season of birth may also play a role in one’s risk for diabetes, obesity, and cardiovascular disease. For example, one study in Denmark reported no connection between season of birth and type 2 diabetes.

One marker for cardiovascular health is heart rate variability, with a higher variability correlated with a lower risk for heart-related problems. In one study on children aged six to 10 in Taiwan, season of birth was related to both sympathetic and parasympathetic activity. Boys with winter birthdays exhibited higher heart rate variability based on higher HF power and lower LF/HF power (balance between sympathetic and parasympathetic activity). They also found that temperature during the season of birth and age were predictive for HF in boys, while humidity during the birth season was predictive for girls. In a study to replicate the findings but with a wider age range and varied geographical areas, men born in winter had higher heart rate variability. In another study, those born in winter demonstrated a greater risk of hypertension, which may be due to a winter pregnancy and increased exposure to air pollution from colder weather.

Some studies have found some association with body mass index (BMI) and season of birth, at least in certain populations. One systemic review found that those born in colder seasons had an increased likelihood of a higher BMI than those born in warmer months. However, the researchers did comment that one study found differing results. Another systematic review concurred that those born in the winter months are more likely to have higher BMI in childhood than those born in March; boys born in March were more likely to have a lower BMI and weight. The review also notes that vitamin D status could be an underlying pathway to the  pattern and should be further explored.

In those with bipolar disorder, season of birth impacted BMI and waist circumference, with those born in the spring having a higher BMI. The study also looked at those with major depressive disorder and did not find the same correlation.

Menarche and Menopause

When a woman is born may also play a role in when she starts and ends menstruation. A study in China found that those born in the spring had an earlier age for both menarche and menopause compared to those born in other seasons. Another study, this one in Poland, found that those born in summer began menstruation earlier than other seasons, especially in the cohort born 1982 – 1984, as the findings for the second cohort born between 1992 and 1994 did not reach statistical significance.

A study in central India collected data from 330 females on age of menarche and found no correlation with season of birth, but the study notes this could be attributed to continuous warm weather and sunlight exposure. A review study published in 2019 discussed numerous childhood factors that affect natural menopause age, including that those born in March are more likely to have an earlier menopause than those born in October.

Cancer

Another area of health that may be impacted by season of birth is cancer risk. Based on one study from China, those born in winter may have a lower risk of developing lung cancer and squamous cell carcinoma than those born in other months. There was an odds ratio of 1.06 for those born in the spring, 1.07 in summer, and 1.06 in autumn, as compared with winter-born individuals.

In a case-control study with 2296 patients, there was a higher incidence of surgery for colorectal cancer for those born in autumn, especially September, while there was a lower incidence in those born in spring. However, gender also played a role, with different seasonal peaks based on sex. In females, there was another peak in February and a lower incidence in summer, while men experienced another peak in June.

One retrospective study on data from the Beijing Institute for Cancer Research found that those born in the summer have less of a risk of developing gastric cancer. The study attributes the variance in immune function to the season, light exposure during pregnancy, and maternal nutrition status. A follow up study published in 2020 on individuals born in Sweden from 1950 to 2014, with a total study population of close to six million individuals, found no correlation of risk associated with those born in the spring.

Mortality and Other Health-Related Factors

There are other areas of health that have been investigated for links between season of birth and health. One UK study found that season of birth was associated with certain growth markers, including birth weight, adult height, and age at menarche although it was not associated with BMI. Those born in summer had later puberty development, were taller as adults, and had a higher mean birth weight, and those born in the winter were the opposite.

Another study looked at mortality differences based on birth month. It found that men born in the fourth quarter had an 11% less chance of death compared to those born in the first quarter. For women, it was more beneficial to be born in the third quarter; they were 14% less likely to die than those born in the first quarter.

A study on patients with atrial fibrillation found that males born in spring and summer had a higher mortality rate, but not females.

Season of birth may also impact the efficacy of vaccines, at least for the rotavirus. One study investigated rotavirus hospitalization in children before and after the implementation of the vaccine. Prior to vaccination, there was little difference between season of birth and risk of hospitalization for rotavirus infection. However, after the implementation of the vaccine, those who were born in winter and had 2 or more doses of the vaccine were better protected against the virus than their spring, summer, or fall counterparts. However, winter births also had the highest hazard ratio of hospitalization, especially if born in odd calendar years (high infection years).

Vision and eye health may also correlate to birth season. A study in Italy found that those born in summer had a 2.5% higher risk for age-related macular degeneration, while those born in winter had a 3% lower risk. Those born in January had the lowest risk, with a 5.9% lower compared to the expected number.

Condition	Winter	Spring	Summer	Autumn
Multiple Sclerosis		Increased		Reduced
Celiac Disease (in children <15)	Reduced	Reduced	Increased	Increased
Allergies		Increased	Increased
Asthma		Decreased		Increased
Schizophrenia	Increased	Increased
OCD		Reduced		Increased
Bipolar Affective Disorder		Increased		Increased
ADHD			Increased
Dyslexia				Increased
Heart-Related Problems	Decreased	Increased	Increased
Obesity	Increased	Increased
High Blood Pressure	Increased			Increased
Menarche		Earlier onset	Earlier onset
Menopause		Earlier onset and more symptoms	More symptoms	Fewer symptoms
Lung Cancer	Reduced
Colorectal Cancer	Increased in women	Reduced	Reduced in women and increased in men	Increased
Gastric Cancer			Decreased
Melanoma		Increased		Decreased
Non-Hodgkin’s Lymphoma		Increased	Increased
Macular Degeneration	Decreased		Increased
Growth Factors	Early puberty, shorter adults, lower birth weight		Later puberty, taller adults, higher birth weight

This is not an exhaustive look at all the studies out there, but I did touch on many of the larger areas that researchers are currently exploring. Although the current research merely points to correlation rather than causation or potential mechanisms, it does demonstrate an intriguing, emerging addition to some of our understanding about disease risk, the human body, and outside influences on health.

Before deciding you are at risk because you were born at a certain time of year, remember that many of these studies are done on particular populations…and that the seasonality and the environment in which the studied population lives may be vastly different than your own. For example, the seasonal impact on those living at high latitudes, such as the studies from Denmark, may be totally different from those living at lower latitudes, closer to the equator, especially if vitamin D plays a big role.

What can you do about this knowledge? At this point, not much more than simply having interesting tidbits for dinner parties. However, while you cannot do much to change the time you were born, you can take action on creating a healthy lifestyle that mediates many of the risk factors for developing chronic illness.

Remember, chronic illness development is generally highly complex with no one trigger or reason for development. By addressing as many of the potential triggers as possible, you can mitigate some, if not all, of the higher risk you may have due to the date of your birth. This can be done in part by modifiable lifestyle factors, such as consuming a colorful, plant-based, whole-foods diet, reducing stress, exercising, and getting sufficient sleep.

If you are concerned about your risk, talk with your doctor or another healthcare practitioner to determine the best strategies for your own situation.