A recent study published in the journal Heliyon found that females born in the summer were associated with improved childhood growth and development as well as adult body size and better educational attainment.
It is widely accepted that vitamin D status among individuals varies depending on the time of the year. For example, in the northern United States, such as New York City or Boston, one cannot produce adequate vitamin D from November through March. This results from the angle at which the sun’s rays enter the Earth’s atmosphere, thereby blocking UVB exposure and preventing vitamin D production from taking place in the skin causing a “vitamin D winter.”
Fetal origins of disease (FOAD) is a theory stating that the risk of developing diseases as an adult is greatly impacted by the events that take place during early development. Since the vitamin D status of the mother directly affects the development of the fetus and such levels are directly passed on to her infant, researchers have hypothesized that the season of birth may contribute directly to the development and health of the infant later on.
Extensive research has been conducted to further understand this relationship. One study found that infants born in the warmer months had improved gross motor skill development compared to those born in the fall. Other research has discovered that the season of birth is associated with immune related diseases, psychiatric conditions, elevated BMI and an increased risk for all-cause mortality in adulthood.
With the implication that the relationship between the season of birth and various health outcomes is based on the seasonal changes in vitamin D status, researchers recently aimed to determine if there is also a link to birth weight, female pubertal timing and adult height and body mass index (BMI).
A total of 452,399 Caucasian participants born in the UK and Ireland were included in the study. Maternal sun exposure was estimated utilizing data from Met Office, which is the national weather service for the United Kingdom. The researchers averaged each individual’s recorded hours of sun exposure per month for the full duration of pregnancy and for three months after birth. The data was then categorized into four groups: the three trimesters of pregnancy and the 3-month period after birth. The individual’s health outcomes were determined via self-reported health questionnaires and anthropometric assessments.
Here is what the researchers found:
- Birth weight, age at menarche and height were significantly associated with season of birth (P < 0.001).
- Those born in the summer (June—August) had an increase in average birth weight (p = 8 × 10−10), were taller and reached menarche at a later age (P = 1.1 × 10−45) compared to those born in all other seasons.
- For each of these traits, maternal sun exposure at the second trimester appeared to have the most significant effect on health outcome.
- Individuals born during the winter (December—February) tended to have a lower birth weight, a shorter stature and reach menarche at an earlier age.
- Those born in February were 23% more likely to have low birth weight than those born in September (P = 4.4 × 10−10).
- Individuals born in September were 1.22 times more likely to go through puberty early compared to those born in July (P = 7.3 × 10−15).
- Those born in December had a 9% increased odds of a shorter height compared to those born in June (P = 0.006)
- Individuals born in the autumn were more likely to continue their education after 16 years of age as opposed to those born in the summer (P = 1.1 × 10−91).
- Those born in the autumn we also more likely to attain a degree level certification than those born in the summer (P = 4 × 10−7).
- Unlike all other health outcomes, both findings on educational success appeared to surround the start of the school year.
The researchers concluded,
“In this large study of ∼500,000 UK individuals, we describe the most comprehensive assessment to date of the impact of birth season on childhood growth and physical development.”
“Our findings provide support for the ‘fetal programming’ hypothesis, refining and extending the impact that season of birth has on childhood growth and development. Whilst other mechanisms may contribute to these associations, these findings are consistent with a possible role of in utero vitamin D exposure.”
This study provides further support to the association between the season of birth and health outcomes later in life. The large sample size increases the strength of these findings.
There are also a few limitations to note. Due to the cross-sectional design, only association was proven, not causality. Vitamin D status was not measured, leaving further room for questioning if vitamin D levels are the link between the season of birth and the health outcomes later in life. Also, most of the measures were self-reported, potentially resulting in recall bias.
Sturges, M. & Cannell, JJ. Closer look: recent research finds season of birth relates to adult health. The Vitamin D Council Blog & Newsletter. October 16, 2015.