Are Scorpios destined to have more children than Virgos — their Earthy opposites? Does being born in the early spring put you at greater risk of cardiovascular disease than having a birthday in the summer?

The Zodiac may not be able to predict your soulmate, but, when it comes to health outcomes, there seems to be some scientific truth behind the astrological musings of the Ancient Greeks. Though it’s likely the environment (of both mother and baby), rather than the stars, that controls your fate.

Mary Regina Boland, PhD, an assistant professor of Informatics at the University of Pennsylvania’s Perelman School of Medicine, is delving deeper into how a birth date may influence someone’s life. Her work began in 2015, when, as a doctoral researcher at Columbia University, Boland led a study investigating the link between a person’s birth month and their risk of various health conditions.

“I was looking at schizophrenia and its potential relationship to Vitamin D exposure, and there were a number of different papers linking different single diseases and birth season. It made me wonder how many diseases have some type of birth-season component,” Boland said.

To find out, she analyzed data from the birth dates of 1.7 million patients treated over 14 years in New York City and found 55 diseases that correlated with specific birth months. The paper, published in the Journal of the American Medical Informatics Association (JAMIA), was one of the first to make the connection between a large variety of diseases — including cardiovascular, respiratory, and neurological — and the time of year a person was born.

Following that research, Boland published another JAMIA study in 2017 that analyzed the medical records of 10.5 million individuals at six sites in three countries — the United States, South Korea, and Taiwan. Beyond the links between disease and birth month, Boland wanted to find out the main drivers behind these associations. That is when she was able to attribute birth-season disease risk with specific factors, such as climate and pollution. For example, the study’s findings included: links between first-trimester exposure to carbon monoxide and increased risk of depressive disorder; first-trimester exposure to fine air particulates and increased risk of atrial fibrillation; as well as decreased exposure to sunlight during the third trimester and an increased risk of type 2 diabetes. 

During the course of that research, Boland saw that multiple studies had made an association between a woman’s birth month and her fertility later in life.

“The main hypothesis was that, if you’re female, being born during higher temperature seasons would make you less fertile than your peers born in winter months, because higher temperatures have been correlated with higher losses of immature ovum cells — basically, that heat destroys your egg count, even as an infant,” Boland said. “This has been shown in countries around the world.”

However, that theory never quite added up in Boland’s mind. “Researchers had found patterns between women born in hot months and having fewer pregnancies,” she said. “But it just seemed strange to me.”

In her latest study, published this month in Nature’s Scientific Reports, Boland found that the association between birth season and fertility is much more nuanced. The new paper is an analysis of 22 fertility studies from around the world that focused on how temperature, as well as other social and environmental factors, could influence the relationship between birth season and fertility.

Boland and her colleagues found that the picture is more complex than any single had study previously revealed; it’s not just heat, but a unique mix of factors that can determine a woman’s number of pregnancies. There does in fact seem to be some relationship between temperature (birth season) and fertility, the study says, but only in certain parts of the world. When assessing other factors, the picture became even more murky. For instance, rainfall at the time of a woman's birth seems to increase fertility — but only in higher altitude locations, like New Zealand, Romania, and Northern Vietnam.

The research may not paint a crystal clear line between temperature and fertility, but it does show that environmental exposures seem to play a role later in life. And considering that more than 12 percent of reproductive age women (15–44 years) in the United States suffer from infertility, the topic is worth taking an even closer look into, Boland says. As climate change continues to impact the world, environmental shifts could, in theory, decrease birth rates throughout an entire region or country.

“I think the major takeaway is that environmental exposures can affect your body in ways that you may not expect,” Boland says. “It’s very possible that climate change could affect how many grandchildren you’ll have. That’s something you’re probably not thinking about.”