Does Your Birth Month Affect How Healthy You’ll Be?

Updated Oct. 8, 2024 at 6:00 p.m.*

BERLIN — Why do some people have high blood pressure, and others asthma? Why are some people prone to psychosis or have ADHD? And why are there certain people who seem to always have a cold, and others never get sick? How healthy we are in life depends on our genes and our lifestyle, of course. On luck and bad luck and where we live. On gender, education and income.

But that’s not all: there is growing scientific evidence that the time of the year of our birth could also play an important role. Studies, for example, show that children born in spring are more likely to develop multiple sclerosis as adults, while more winter children develop schizophrenia. Summer children are more likely to have asthma and autumn children are more likely to suffer from allergies.

And science now has some answers to the question of why this could be the case. As so often happens in science, it’s also a puzzle. So let’s take a look at the pieces.

The fact that predisposition to a certain disease is more common in people born in certain months is apparently linked to the conditions to which both we and our mothers were exposed to during pregnancy and in the first weeks of our lives. And these differ depending on the time of year.

In order to find out exactly what role the season plays, scientists are trying to work out all the other influences on our health as best they can and are looking for clues in statistics and biobanks. Which season-specific conditions could cause people born in the same month to develop the same problems later on? Is it the temperature? The sunlight? The air quality? The different eating habits in summer or winter? Let’s explore the latest findings.

Respiratory illnesses 

For example, respiratory pathogens such as influenza, rhino and respiratory syncytial viruses (RSV) mainly circulate in the fall and winter here in Germany and elsewhere in the Northern Hemisphere. This can have a lasting impact on the health of children born during this period — both through direct transmissions and because they affect the mother during pregnancy.

Early contact with germs strengthens the immune system.

Unlike spring and summer babies, children born between November and February often have their first mild respiratory infection at just a few weeks old. This usually results in a stuffed nose and fever, but otherwise has no consequences. It is often even said that early contact with germs strengthens the immune system and that winter babies are therefore less likely to catch a cold later on.

But unfortunately it’s not that simple. What’s more, some infections are not so harmless. A severe RSV infection in the first year of life, for example, has been shown to increase the risk of asthma.

A 2023 study published in the journal Lancet found that children who do not contract RSV in infancy have a 26% lower risk of developing asthma by their fifth birthday. The scientists also found that 15% of asthma cases in under-fives could be avoided if the children were not infected with the RS virus in the first year of life.

According to the Robert Koch Institute, the peak of the RSV season in Germany lasts four to eight weeks and is usually in January and February. And studies actually show that people born during these months are more likely to suffer from asthma in childhood and adulthood than others.

Fortunately, as long as a baby is still in the womb, it is protected from most bacteria and viruses. However, the respiratory pathogens that circulate more in the fall and winter can already have an indirect impact on their health. In the past, this was demonstrated by the Spanish flu, which was rampant between 1918 and 1920, and which also affected many pregnant women — they are generally more susceptible to infections because their immune system changes during pregnancy.

U.S. researchers compared the birth cohorts from this period with the cohorts before and after and found that children whose mothers had been infected with Spanish flu during pregnancy had a more than 20% higher risk of cardiovascular diseases such as high blood pressure and diabetes between the ages of 60 and 82.

Mosquito bites

Although the risk of respiratory infections is significantly higher in the fall and winter, there are other pathogens that can infect pregnant women and babies and that occur almost exclusively in the warmer months. We are talking about viruses such as dengue, which can be transmitted by mosquitoes of the genus Aedes.

Due to climate change, dengue fever in particular no longer only occurs in the tropics and subtropics, but has also been spreading to the rest of the world in recent years. In Europe, there have already been outbreaks at Lake Garda and in Paris. Scientists assume that a vector of dengue fever, the Asian tiger mosquito Aedes albopictus, could soon become native to more northern regions.

If an infant contracts dengue fever, it usually recovers completely. In rare cases, however, a severe dengue infection at a very young age can also have long-term effects. For example, the immune system of these children can suffer, making them more susceptible to infections in the first years of life and also to a second dengue infection. If a pregnant woman is infected with dengue, the risk of premature birth can increase — which in turn often has consequences for the child’s later health.

Daylight

In northern locations, the body often produces less vitamin D than we need during the darker months of the year.

Vitamin D acts like a hormone in the body and is responsible, among other things, for ensuring that the bones absorb sufficient calcium and thus gain hardness. While babies in the first year of life are recommended vitamin D supplements, the nutrient is also important for unborn babies so that their skeleton can form properly and their immune system can mature.

The last trimester is crucial.

If a child does not receive enough vitamin D in the womb, this could raise the risk of contracting multiple sclerosis (MS), among other things. This is at least how some researchers explain the fact that children born in spring are more likely to develop MS than children born in fall, for example. Their mothers probably received too little sunlight over a longer period of time during pregnancy.

A vitamin D deficiency before birth could have an effect on mental health. In recent decades, scientists have repeatedly observed that winter children have an increased risk of developing autism, schizophrenia or psychosis. One of the main hypotheses: insufficient vitamin D supply in the womb.

But mothers of winter babies were pregnant during the summer and were able to get plenty of daylight during this time. So why should there still be consequences due to a vitamin D deficiency? Some scientists believe that the last trimester — when it was already darker outside and the sun had lost its strength — is crucial. It could represent a particularly sensitive phase of brain development.

This theory could lead us to believe that summer children are happier than winter children and are less prone to depression as adults. But no such correlation has been shown to date; individual studies even suggest that the opposite could be the case.

The exact role that vitamin D plays during pregnancy is generally disputed, so doctors do not recommend pregnant women to take vitamin D supplements unless they are diagnosed with a vitamin D deficiency.

Weather

For decades, the weather has become significantly warmer due to climate change. Periods of extreme heat are also increasing. Unlike people in Africa, South America or South Asia, for example, Northern Europeans are not used to such temperatures, which is why our bodies react more strongly to them — including the bodies of pregnant women, of course. Temperatures of up to 30 °C (86 °F) for five consecutive days already increase the risk of a premature birth in this country, as a study by Hamburg researchers from 2023 shows. If a baby is born prematurely, this can have multiple consequences for its health.

But the climate is not only getting hotter and hotter, extreme weather events such as severe thunderstorms, heavy rain and storms are also on the increase. A baby in the womb hardly notices any of this, you might think, after all it is floating relaxed in 37-degree (98.6F) amniotic fluid.

But a study by Canadian scientists suggests that girls whose mothers had experienced the great ice storm in the province of Quebec in January 1998 during pregnancy were more likely to develop asthma later on. The researchers assume that the storm caused severe stress in the expectant mothers. This in turn changed the immune system of the unborn babies in such a way that it promoted asthma. Other studies on the subject confirm that stress can have a general effect on children in the womb: according to these studies, psychological stress can not only trigger premature births, it can also impair the baby’s brain development, for example.

What mothers eat at Christmas

We know the mother’s diet during pregnancy is one of the most important factors that determines how healthy we will be later in life. For example, if a pregnant woman takes in too few nutrients through her diet, this can have a lasting effect on the development of the foetus. A folic acid deficiency, for example, can lead to neural defects such as an open spine, which is why expectant mothers in this country are given folic acid tablets as standard.

Studies indicate that people in Europe consume different amounts of nutrients throughout the year. According to a study by the Max Rubner Institute, for example, more vegetables are eaten in Germany in summer than in winter. In the cold season, people eat more citrus fruit such as oranges, but also more sweets such as gingerbread, cookies and chocolate.

Scientists from Germany have found the fact that people overeat at Christmas as a possible reason why May children in this country have a higher cardiovascular mortality rate than November children. The researchers examined data from more than six million deaths from cardiovascular disease and found that subjects born in May died significantly earlier than those born in November — by an average of nine to ten months. In addition to maternal nutrition, they cite the weather, exposure to sunlight, air pollution and infectious diseases as other possible causes.

Complicated puzzle

Anyone reading these findings may get the impression that, in terms of health, no season is particularly good for giving birth. It is probably a good thing that it’s not that simple to determine “a better time to be born.”

It is one of many influencing factors.

Still, it is also true that the correlations described are not random; they are statistically significant. But just because spring babies seem to be more allergic later in life does not mean that all people born in spring get hay fever later. And just because winter children seem to have asthma more often later in life and just because severe RSV infections promote asthma does not mean that they are always the reason why a person born in winter develops asthma. It is likely that there are other factors playing a much bigger role: whether the mother or father already suffers from asthma, whether one of them smokes or whether the parental home is located on a busy road.

Secondly, many scientists do offer explanations in their studies that seem quite logical. It is easy to imagine, for example, that a baby whose immature immune system is exposed to a lot of pollen shortly after birth will later have an allergic reaction to it as an adult. But whether this is really the case and, if so, what happens in the body in detail is not yet well documented.

Overall, the month of birth should be seen as one of many influencing factors, a small piece of the puzzle that at best helps you to better understand your own health as a whole.

*Originally published Sept. 23, 2024, this article was updated Oct. 8, 2024 with enriched media and an Extra! box about Finland.