Very much a niche topic in allergy, some consider birth month a risk factor, especially in children. This so-called ‘horoscope’ effect (1), the idea that birth month influences allergy risk, has been around since at least the 1950s (see tables below). Many, not all, of the studies listed in these tables are referred in reference.
Trawling through these peer-reviewed studies reveal several notes of caution
- Studies may be subject to , i.e., those showing positive outcome are more likely to be published.
- Many studies weren’t properly controlled, i.e., didn’t use non-allergic subjects but rather often used birth records from among the general population as controls.
- Many studies lacked objective criteria for defining allergy such as skin prick tests or circulating serum IgE antibody levels. Instead they relied on self-reporting.
- Many studies were based on questionnaires or were retrospective, i.e., subject to .
- Questionnaire-alone based studies had an additional flaw since they didn’t perform allergy test on subjects themselves thus amplifying hindsight bias. Even if previous tests were included in such analyses, authors of such studies couldn’t vouch for the accuracy of such tests and their data.
Some studies found birth month influence on allergy risk wanes with age.
- In a 1980 study, birch pollen allergy risk was higher in those born between February and April from among 2171 Finnish children (3). Risk differences flat-lined by the age of 20 years.
- In a 1990 study, atopic and non-atopic rhinitis was more likely in those born in May to June among 327 adults in southwest London (4). Again, risk differences flat-lined by the age of 20 years.
One study found a birth month allergy risk association for 18 year olds born in 1963 but not for those born in 1953 (5). What change over that decade could explain this? Dramatically reduced early life exposure to microbes somehow linked to allergy promoting environmental variations that change seasonally? This leads us to other factors that could reduce birth month risk. For e.g., its influence on seasonal allergic rhinitis is minimized in large families (). This effect of course alludes to the health beneficial value of exposure to dirt/microbes early in life, i.e., .
Regardless the various weaknesses attendant to this body of work, how does birth month influence allergy risk? Several hypotheses have been suggested
- Birth during months of peak seasonal allergens (e.g., pollens) could predispose such infants to develop allergies against them, according to those researchers who assume newborn immune systems aren’t yet fully developed. However, this can’t explain birth month connection to non-seasonal allergens, i.e., food, house dust mite, cat, dog.
- Seasonal sun (UV) exposure and attendant vitamin D levels appear to influence the nature of the immune response. With respect to allergies, the notion is birth during months of low sun exposure, i.e., autumn and winter, predispose such infants to transient vitamin D deficiency at a time their developing immune function is most vulnerable to its effects ( , , 9, ). According to this hypothesis, adequate vitamin D levels early in life help the developing immune system ‘mature’ properly to effectively manage tissue inflammation and infections ( ). After all allergies are a sign of dysregulated immune function. However, since allergies have only really skyrocketed over the last 50 years or so while seasonal reductions in sun exposure have been a mainstay in temperate regions, this hypothesis can’t be a stand alone in explaining why allergies are increasing.
- One hypothesis posits drier skin in winter combined with reduction in access to the ‘protective effect’ of sun exposure weakens skin barrier function in newborns born in autumn/winter. Such authors claim this could plausibly explain the connection between birth month and eczema/atopic dermatitis (9).
Bottomline, it’s far from clear if and how birth month influences allergy risk. A thorough, rigorous meta-analysis of data generated in this sub-field is very much overdue. A better delineation of latitude effects is also warranted though this may be diluted in places where vitamin D supplementation is becoming commonplace.
1. Aalberse, R. C., et al. “‘Horoscope effect’ not only for seasonal but also for non‐seasonal allergens.” Clinical & Experimental Allergy 22.11 (1992): 1003-1006.
2. Wjst, Matthias, et al. “Latitude, birth date, and allergy.” PLoS Med 2.10 (2005): e294.
3. Björkstén, F., I. Suoniemi, and V. Koski. “Neonatal birch‐pollen contact and subsequent allergy to birch pollen.” Clinical & Experimental Allergy 10.5 (1980): 585-591.
4. SIBBALD, BONNIE, and ELIZABETH RINK. “Birth month variation in atopic and non‐atopic rhinitis.” Clinical & Experimental Allergy 20.3 (1990): 285-288.
5. Åberg, N. “Birth season variation in asthma and allergic rhinitis.” Clinical & Experimental Allergy 19.6 (1989): 643-648.
6. Bousquet, Jean, et al. “Allergic rhinitis and its impact on asthma (ARIA) 2008*.” Allergy 63.s86 (2008): 8-160.
7. Vassallo, Milo F., et al. “Season of birth and food allergy in children.” Annals of Allergy, Asthma & Immunology 104.4 (2010): 307-313.
8. Pyrhönen, Kaisa, et al. “Season of the first trimester of pregnancy predicts sensitisation to food allergens in childhood: a population-based cohort study from Finland.” Journal of epidemiology and community health (2010): jech-2009.
9. Mullins, Raymond James, et al. “Season of birth and childhood food allergy in Australia.” Pediatric Allergy and Immunology 22.6 (2011): 583-589.
10. Mullins, Raymond James, and Carlos A. Camargo. “Latitude, sunlight, vitamin D, and childhood food allergy/anaphylaxis.” Current allergy and asthma reports 12.1 (2012): 64-71.
11. Keet, Corinne A., et al. “Potential mechanisms for the association between fall birth and food allergy.” Allergy 67.6 (2012): 775-782.