There's a good reason to be worried if your partner is nearing a milestone birthday

At some point during adulthood, birthdays become bittersweet occasions.

Sure, it's a chance to have a second piece of cake because "calories don't count" today, but many of us also smack up against the realization that we haven't accomplished everything we'd hoped in the last 12 months.

The question is: How do we alleviate this disappointment? Certainly, some of us just mope, but others take drastic measures.

A 2014 study found that men are more likely to cheat on their partner when their age ends in the number nine, i.e. when they've got a milestone birthday coming up.

For the study, the researchers looked at more than 8 million users on Ashley Madison, a site for people seeking extramarital affairs. As The Wall Street Journal reported, 950,000 men reported ages ending in nine, meaning the likelihood of a man nearing a milestone birthday winding up on the site was 18% higher than chance levels.

According to The Wall Street Journal, the researchers found a similar, but weaker, pattern among women.

The authors suspect these findings have to do with people's increased tendency to seek existential meaning around big birthdays. Two other studies the researchers conducted, published in the same paper, found that people are more likely to think about meaning and purpose in life when their age ends in nine.

Of course, a person's age is rarely the real reason they cheat, even if it is the impetus. If you're unhappy in your relationship, you might not appreciate that until you're about to turn 40 - but turning 40 isn't why you're dissatisfied.

Remember that our expectations for marriage have increased considerably over the last few decades, meaning that we now tend to turn to our partner for personal fulfillment. If you're feeling somewhat unsatisfied, your relationship might not be the culprit - or at least the only culprit. Consider tapping into your friendships, work, and hobbies to see what changes you can make in those areas.

Perhaps simply being aware of the existential crisis looming ahead can prevent you from taking a risk that could potentially ruin your relationship.

Knowing that you've got a milestone birthday coming up, you might want to take a moment to reassess: What is it that you really want and how can you get there? Then you might share those insights with your partner and figure out an action plan together.