Female Reproductive Performance and Maternal Birth Month: A Comprehensive Meta-Analysis Exploring Multiple Seasonal Mechanisms

Abstract

Globally, maternal birth season affects fertility later in life. The purpose of this systematic literature review is to comprehensively investigate the birth season and female fertility relationship. Using PubMed, we identified a set of 282 relevant fertility/birth season papers published between 1972 and 2018. We screened all 282 studies and removed 131 non-mammalian species studies on fertility and 122 studies that were on non-human mammals. Our meta-analysis focused on the remaining 29 human studies, including twelve human datasets from around the world (USA, Europe, Asia). The main outcome was change in female fertility as observed by maternal birth month and whether this change was correlated with either temperature or rainfall. We found that temperature was either strongly correlated or anti-correlated in studies, indicating that another factor closely tied to temperature may be the culprit exposure. We found that rainfall only increases fertility in higher altitude locations (New Zealand, Romania, and Northern Vietnam). This suggests the possibility of a combined or multi-factorial mechanism underlying the female fertility-birth season relationship. We discuss other environmental and sociological factors on the birth season-female fertility relationship. Future research should focus on the role of birth season and female fertility adjusting for additional factors that modulate female fertility as discussed in this comprehensive review. Over 12% of reproductive age women (15-44 years) in the United States of America suffer from infertility or impaired fecundity 1 , with some estimates as high as 16% 2. European countries show similarly high infertility rates with Great Britain at 17% 3. In developing countries, the rate of infertility is estimated at 1 in 4 couples 4,5. In addition to rising infertility rates among humans, increases in infertility and reproductive disorders were observed in dairy cows 6. Dairy researchers found that a cool, low-temperature, environment appeared to preserve fertility and reduce the risk of reproductive disorders 6 furthering support for a relationship between temperature and female fertility. Although at least one study points to other factors besides temperature that play a role more significant role at the country-level on female fertility 7. Maternal birth season effects female fertility outcomes 8 , including in both hunter/gather societies (e.g., Hiwi of Venezuela) 9 and industrialized societies 8,10. Birth season can affect later risk of disease through alterations in the prenatal environment that alter growth patterns and processes 11. In addition, the known relationship between temperature and fertility 6 could also manifest itself in a birth season relationship. The main biological mechanism underlying a birth season relationship with female fertility remains the hypothesis that oocytes' exposure to high temperatures at birth and shortly thereafter results in increased oocyte

Reproductive seasonality in the Baka Pygmies, environmental factors and climatic changes

“…We were not able to distinguish whether these are independent effects or whether they arise because of the co-linearity with the climatic variables. In both cases, however, there is a combined mechanism at work between temperature and rainfall, as found by others [3]. In this sense, temperature might not correlate directly with conceptions and births (via biological or behavioural mechanisms) but it might have a role in controlling other variables that do affect the reproductive pattern.…”

“…the seasonality of conception and birth, is of importance because birth months have significant consequences for many demographic aspects such as infant mortality [e.g. 1 , 2 ], maternal performance [ 3 ], adult height [ 4 ], lifespan [ 5 , 6 ] and susceptibility to late-onset diseases [e.g. 7 – 9 ].…”

Harnessing Electronic Health Records to Study Emerging Environmental Disasters: A Proof of Concept with Perfluoralkyl Substances (PFAS)

“…This enabled us to develop a method that correlated birth season and trimester information with climate and pollution variables using location information 34 . We have also used our methods to study the effects of social exposures that can manifest as birth month relationships for Attention Deficit Hyperactivity Disorder (ADHD) 35 and increased the number of datasets included in our assessment of female fertility and birth season to assess deeper and more complex multi-factorial environmental exposures 36 . We have found that prenatal exposure to fine air particulates increased the risk of atrial fibrillation later in life in humans 34 ; this finding was supported by a study in a canine population 37 .…”

Maximum apparent temperature lowers birth weight and increase hazards of preterm birth in a constant high ambient temperature exposed population in Piura, Peru

“…In addition, a number of unusual weather events have affected the quality of life of people around the world, posing challenges to the health systems of many countries (2). For example, rates of diarrhea and vector-borne diseases have increased in tropical locations and hot climate regions (3,4), also affecting the reproductive output of affected populations (5).…”