Exactly four years after a British mother gave birth to a pair of twin boys, on July 18, she delivered a second set of naturally conceived twins, according to a report Thursday in the British newspaper The Sun. The mom, Kim Hefer, is reportedly the first woman in the United Kingdom to have to two sets of twins on the same day years apart. A bookie set the odds of this happening at 30 million to one, the paper reported.

That’s only the most recent case of attention-grabbing coincidental births. A mother, daughter, and granddaughter all sharing the same birthday or twin sisters giving birth on the same day or a boy and girl born on the same day in the same hospital who grow up and decide to get married -- these odds-defying stories capture our imagination.

What is it about shared birthdays that are so irresistible?

Humans understand the world exists in certain ways; for example, we know that weather patterns, not angry gods, cause thunder. But something we perceive as coincidental, say a mother, daughter, and granddaughter sharing the same date of birth, seems so random to us so we believe that supernatural forces cause this coincidence.  

“Basically any event is unlikely. If you flip a coin five times and it comes up heads each time, you think that something funny [has happened]. The probability of getting heads, heads, heads, heads, heads, is the same as any other sequence. It’s not that it is unlikely … there is something else about it that strikes us,” says Tom Griffiths, director of the Computational Cognitive Science Lab and the Institute of Cognitive Brain Sciences at the University of California, Berkeley.

Griffiths and his colleague wrote a paper, which explores coincidences and also considers “the birthday problem.” A classic statistical exercise, the birthday problem aimed to discover how many people needed to gather in a room for two to share a birthday. If there are only 23 people in a room, there is a 50 percent chance that two share a birthday and this likelihood increases as the number of pairs entering the room grows. Griffiths’ findings reinforce what experts know about the birthday problem—shared birthdays aren’t as random as we like to think.

“Why is it that people having the same birthday is something that strikes us? Is it that it suggests there is some underlying reasoning behind it? Our brains want to believe that it is something other than chance,” Griffiths says.

Atlanta psychologist Robert Simmermon agrees that people assign meaning to random events, believing God, fate, karma, or the stars contribute to chance.      

“One in 30 million chances … that chance is incomprehensible,” says Simmermon, who has a private practice. “We really can’t understand anything else that might be like it.”

And people feel more intrigued by these birthday coincidences because birthdays carry a lot of emotional weight.

“It’s the genesis; it is the beginning,” he says. “The birthday is the beginning of our existence … well, the beginning of our consciousness.”

Do you share the same birthday as a sibling or other close family member? Share your unusual birthdate coincidences on Facebook

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Besides the cards, cake, candles, and gifts, there might be one more unexpected surprise in store for you on your birthday -- a higher risk of dying.

A new study suggests that men and women are 14 percent more likely to die on their birthday than any other day of the year.  This effect was strongest in people aged 60 and older, who were up to 18 percent more likely to die on their birthday.

"We find more deaths than expected solely on birthdays, that is neither in the days before nor after the birthdays," says lead author Dr. Vladeta Ajdacic-Gross, an epidemiologist for the Institute of Social and Preventive Medicine at the University of Zurich.

In this large study, which was published in the Annals of Epidemiology, researchers analyzed data from more than 2.4 million Swiss people who died between 1969 and 2008. They tried to determine if there was any relationship between when people died and the cause compared to their birthdays.

"We were surprised that our findings apply to specific causes of death," says Ajdacic-Gross. Both violent causes, such as suicides and falls, as well as strokes and cardiovascular deaths were tied to birthdays. "In addition, older people are distinctly more vulnerable than younger people," he adds.

Men were more likely to have a violent demise on their birthday, as their chances of suicides spiked by nearly 35 percent and their odds of being accident victims rose by almost 29 percent. Deadly falls in men rose by 44 percent on their birthdays, and, interestingly, the number of falls started to increase about four days before the celebration took place.

When it came to natural causes of deaths, women were at greater risk -- almost 22 percent -- of passing away from a stroke on their birthday. For both ladies and gentleman, there was a nearly 19 percent increase in dying from heart-related reasons.

Scientists also unexpectedly found the risk of dying from cancer jumps by 10.8 percent on birthdays. Cancer deaths also seemed to be higher than usual in the days immediately following a birthday -- a trend not seen for any other cause of death. 

But the question you may be asking is: Why? Is birthday partying to blame? Is it a psychological let-down to realize you're one year older -- and not getting any younger?

"We can only speculate on the reasons for the extra deaths on birthdays," points out Ajdacic-Gross. They suspect that birthday-related stress in older people may play a part in the jump in stroke and cardiac deaths.

Other studies have suggested that alcohol may be a reason behind the spike in suicides and perhaps even falls.

The Swiss researchers say their results support the theory of an "anniversary reaction" or "birthday blues," meaning that deaths occurred more frequently on a birthday. They suggest this is a more likely explanation than the "death postponement" hypothesis, or that deaths are less likely to take place on birthdays and people might hang on long enough to reach this yearly milestone -- or to see through certain holidays or special occasions.

Some famous people who have passed away on their birthday include the actress Ingrid Bergman, feminist Betty Friedan, and William Shakespeare. 

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