No matter what we say, we all believe in astrology a little. We all swipe through zodiac posts just to read what they say for our sign. Posts like “what career you should choose based on your birth month” get our attention every time. But did you know your birth month can also hint at what kind of diseases you might face later in life? 

How does birth month affect your health?

The biggest study on this topic came from Columbia University in the US. Researchers analysed the birth dates and medical records of 1.7 million people treated at a hospital in New York between 1985 and 2013. They compared birth months with 1,688 different health conditions to see whether any patterns emerged.

The study found 55 diseases dependent on birth months, most already known, but 16 completely new. Interestingly, more than half of these new links were heart-related conditions, pointing to a connection between heart health, early development, and vitamin D.

Their main conclusion was that birth month does affect lifetime disease risk, likely because of what happens to the body during specific seasons early in development. Your health depends partly on what you’re exposed to in the womb and in your first few weeks of life. Seasons change how much fresh fruit and vegetables are available and how many infections are circulating. They can also influence how much stress a pregnant woman is under. All of these factors can affect a baby’s early health and later development. Scientists have also studied DNA methylation, a set of switches that turn genes on or off. A study based in the Isle of Wight in the UK found a link between birth season and eczema. It identified 92 gene regions connected to allergies that changed depending on birth season.

But the researchers were honest about the limits of their work. The study cannot fully prove that birth month directly causes disease, since a baby born in a certain season may simply be more exposed to infections common in that season.

Winter babies and the immune system

Some of the strongest findings in this field are about immune and allergy-related conditions, and multiple sclerosis (MS) is the most studied example. A meta-analysis of 22 studies and over 145,000 MS patients found significantly more people with MS were born in spring, especially March to May, with the lowest risk for November births, concluding that birth month is genuinely linked to MS risk. A separate review of nearly 80,000 patients found a similar pattern and linked it to latitude, concluding the effect is likely driven by UV light and maternal vitamin D during pregnancy, since it grew stronger further from the equator.

This vitamin D link shows up again in a large UK study of over 115,000 patients that looked at MS alongside five other immune diseases, including type 1 diabetes and lupus. It found risk peaked for people born in April and dipped lowest for October, closely tracking mothers’ likely UV and vitamin D levels during late pregnancy. The researchers concluded this shared pattern across unrelated diseases points to gestational vitamin D as a common underlying risk factor.

Winter birth and the brain

The connection between birth season and schizophrenia has been noticed by doctors. A 2023 study combined 43 studies from 30 countries, covering more than 440,000 people with schizophrenia. It found that people born in winter had a small but real increase in risk, while those born in summer had a lower risk.

What’s interesting for India is that this pattern isn’t the same everywhere. Data from Southern Hemisphere countries has been less consistent than data from the Northern Hemisphere. This suggests that whatever drives the effect may be weaker closer to the equator. It is a useful reminder that these patterns depend heavily on geography.

Diabetes, heart disease, and why India needs its own data

Most of the big studies on this topic come from the US, UK, Japan and China, places farther from the equator than most of India, where winter sunlight is genuinely low. So, does the same pattern hold closer to the tropics?

A study using the China Kadoorie Biobank tried to find out, since China, like India, has both high variations in latitudes. It followed over 461,000 people aged 30 to 79, from 2004 to 2013. It found that those born in spring, autumn or winter had a nine per cent higher risk of developing type 2 diabetes compared to those born in summer. This pattern held for both men and women, in cities and villages alike.

The researchers think summer-born babies likely benefited from better maternal nutrition in the final months of pregnancy, since food variety was traditionally richer in the Chinese summer. This risk didn’t disappear even after accounting for adult body weight, suggesting something is set early in development rather than through people simply growing up heavier or leaner. Interestingly, similar studies in Denmark found no such link. Researchers think this is because Denmark has far less seasonal variation in sunlight and food than China. That may explain why countries such as China and India reveal patterns that do not show up in Europe.

What do we actually know about India?

Large studies directly connecting birth month to disease risk, like those done in the US or China, haven’t been done in India yet on a big scale. But we do have good evidence that the same basic ingredients, like monsoon-related infections, seasonal nutrition gaps, and widespread vitamin D deficiency, are present here too.

Start with vitamin D. Even though India gets plenty of sunshine, vitamin D deficiency is surprisingly common. People spend more time indoors, air pollution blocks sunlight, and darker skin makes it harder to produce vitamin D from sun exposure. A study at AIIMS, New Delhi looked at 98 healthy, breastfed 3-month-old babies, some born in winter and some in summer, along with their mothers. It found deficiency was shockingly common overall, in about 67 per cent of babies and 81 per cent of mothers, regardless of season. Nearly a third of the most deficient babies already showed early, X-ray-visible signs of rickets.

The effects show up later in growth too. A Delhi study followed 912 children who had taken part in a vitamin D trial as babies. It found that stunted children took much longer to run 20 metres and were far more likely to fail basic developmental tests. None of this proves a clear “December baby vs June baby” story for India the way the Columbia study did for New York. However, it explains why Indian researchers are increasingly interested in how seasons and health connect.

These studies are relevant and important. But we should not take them to extremes. Birth month does affect your health, but that doesn’t mean your habits cannot help you maintain a healthy lifestyle.

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