Does your month of birth determine you developing Hashimoto’s and why?

Does your month of birth determine you developing Hashimoto’s and why?

Results of Boost Health science survey, part I

We carry a code to develop a certain health condition(s) in our genes, our DNA. To put it simply, it is passed down to us from our ancestors and the combination of DNA we get from our biological parents is the very first step determining our predisposition to develop almost any disease.

The rest is the doing of our environment: it started determining our chances to develop a disease as early as when we were in the uterus, our home for the first nine months (give or take) of our life.

What does science say?

Research has shown that a season of birth affects development of some autoimmune diseases (multiple sclerosis (MS), type 1 diabetes, rheumatoid arthritis (RA), ulcerative colitis and systemic lupus erythematous (SLE)).

One big study, done on more than 115,000 people, shows that most of the people diagnosed with autoimmune diseases are born in April and the least in October; research was done on people born and living in the Northern Hemisphere (1).

Why does the birth month matter?

Lack of sun exposure and low vitamin D levels are listed as environmental triggers of autoimmune diseases, and the lack of them impacts our health even before we are born. Availability of certain fresh foods changes through seasons and can potentially impact the development of autoimmune diseases (2).

A hypothesis in science and medicine called the FOAD (fetal origin of adult disease) states that environment during our early development (pregnancy) has a deep impact on our later risk of developing adult diseases. There are specific periods during our development where we are more sensitive to our environment, and the environment might cause permanent changes in some of our organs and how they function.

For example, it has been shown that the low birth weight at the full term predisposes people to adult heart diseases, obesity and diabetes (3).

Increasing number of research studies shows that being born during a certain season may determine probability to develop an autoimmune disease later in the life (4–6). A number of research studies have addressed this question some decades ago, but the sample size was too small to draw any firm conclusions (7–12).

Does seasonality only apply to autoimmune diseases?

No, seasonality of birth and predisposition to adult disease is not limited to autoimmune diseases only.

Scientists from Columbia University found 55 out of 1688 diseases to be correlated with the season of birth. For example, the study shows that individuals born in March have a higher chance of developing heart-related diseases and a lower chance of respiratory and neurological conditions (13).

What does our survey show?

Our results follow a pattern of research studies done on other autoimmune diseases: the lowest number of people diagnosed with Hashimoto’s are born from October to February. And the highest number of people diagnosed with Hashimoto’s were born from March to June.

What does this mean?

Sun hours and vitamin D can play a role in development of Hashimoto’s, similar to other autoimmune diseases.

We have extracted a number of sun hours from 10 Northern Hemisphere cities, and pooled them in 3-month periods: December-March (WINTER), March-June (SPRING), June-September (SUMMER) and September-December (AUTUMN).

Then we checked if high sun hours during the late pregnancy (predicted third trimester) correlate with lower rates of Hashimoto’s diagnosis.

We observed that more sun exposure during the predicted third trimester of pregnancy correlates with lower rates of Hashimoto’s diagnosis.

A period between June and September has the highest number of sun hours. People being born starting three+ months later, from October to February indeed have the lowest rate of Hashimoto’s diagnosis.


Autoimmune diseases are coded in our genes, but are triggered by our environment, such as vitamin D deficiency, various infections, environmental pollutants and smoking. The main “source” of vitamin D is through sun exposure, and for that reason it is likely that the increased number of sun hours has a potential to reduce chances of developing an autoimmune disease.

Our survey is relatively small to draw conclusions. We would need more participants to understand if and how the disease is distributed. It would be particularly interesting to see if there is a reverse effect in the Southern Hemisphere. We would also like to research in year of birth impact, especially when meteorological conditions could have been different than the average, for example an overcast and short summer or sunny and warmer than usual winter.

If you are pregnant, or planning a pregnancy, it might be good to talk to your healthcare provider about your vitamin D levels and set a plan on how to maintain it at an appropriate level throughout the pregnancy.

And, being born in a certain “high-risk” month does not mean each and every one of us will develop a disease.


  1. Disanto, et al. Month of birth, vitamin D and risk of immune-mediated disease: a case control study, 2012

  2. Branca F, et al. Seasonality Encyclopedia of Human Nutrition, 2005

  3. Calkins K, et al. Fetal Origins of Adult Disease, 2011

  4. Willer CJ, et al. Timing of birth and risk of multiple sclerosis: population based study, 2005

  5. Kahn HS, et al. Association of type 1 diabetes with month of birth among U.S. youth: The SEARCH for Diabetes in Youth Study, 2009

  6. Rothwell P, et al. Seasonality of birth of patients with childhood diabetes in Britain, 1996

  7. Ekbom A, et al. Is there clustering of inflammatory bowel disease at birth? 1991

  8. Sorensen HT, et al. Does month of birth affect risk of Crohn’s disease in childhood and adolescence? 2011

  9. Buchanan WW, et al. Month of birth and rheumatoid arthritis, 1987

  10. Sonnenberg A. Date of birth in the occurrence of inflammatory bowel disease, 2009

  11. Bai A, et al. Seasonality in flares and months of births of patients with ulcerative colitis in a Chinese population, 2009

  12. Angelucci E, et al. Monthly and seasonal birth patterns and the occurrence of Crohn’s disease, 2009

  13. Boland MR, et al. Birth month affects lifetime disease risk: a phenome-wide method, 2015