If you got engaged and instantly decided to lose weight, there's a big downside.

An engagement ring can trigger an unrecognisable desire to eat healthily, diet crazily and have your body, skin, hair, nails (you didn’t care about pedicures before the diamond) in perfect, photo-ready order for the big day.

Marriage, on the other hand, can create a sense of comfort and freedom that completely undoes all of the aforementioned changed.

Coincidence? Definitely not.

Research out of the Flinders University in Adelaide shows women gain at least 2 kilograms (kg) in the first six months of marriage. The weight gain was more prominent for women who actively tried to lose weight before their wedding (they put on 3.2kg in the six-months following). While women who were pressured to lose weight before the wedding, usually from their fiance or mother, put on an average of 4.5 kg in the first six-months of marital bliss.

Two to four kilograms over six-months does not seem like much (let’s face it; it’s a fluctuation that can happen every menstrual cycle). But, over the long term, these small increments can add up to extreme changes. A 2012 study, published in Obesity, found women in their 20s gain an average of 10 kg in the first five years of marriage, while men in the same age group will put on around 13.5 kg in the same time period. Interestingly, couples who were living together but not married did not show the same pattern.

So what is it about getting hitched that tips the scales?

The goddamn relief.

I don’t know about you, but the pre-wedding workouts / diets / detoxes / beauty plans sound absolutely exhausting (not to mention unnecessary – you weren’t detoxed, fake tanned or living off kale when you met him having tequila shots at the bar, were you? Were you!?)

History aside, however, the pressure to look your best for your wedding is undeniable. I’m sure it comes from an internal place of “I’ve dreamt about this day since I was five, so I better have the same flawless, radiant, I-actually-look-like-Angelina complexion I’ve always envisaged”.

But external factors also play a part. Some women, like those mentioned in the University of Adelaide study, experience direct pressure from family or friends to lose weight before the wedding (manners, anyone?). And any modern bride is likely accounting for the hundreds of eyes that will see, like, comment, judge their wedding photos when they’re inevitably posted to social media. All this expectation mixes into a melting pot of diets, exercise, and juice cleansers that do awful things to your metabolism.

So… What happens when the pressure is off, and the vows have been said? Same thing that happens after your final exams at university, or after an extreme deadline at work. You cut loose. The photos have been taken, the guests have been wowed, you’ve lived up to the expectations of your five-year-old self. It’s time for pasta and potato chips.

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The honeymoon phase.

If there is a ever a time to treat yourself, you’d think your wedding and honeymoon would be it.

Celebrating and holidaying go hand-in-hand with food and drink, and it’s easy to talk each other into bad (but very delicious) life decisions on when you’re an invincibly-happy pair of newlyweds.

“We’ve just been married, we’re allowed that gelato, or that second serving of pizza, as well as that irresistible dessert. This is OUR time.”

If your honeymoon phase is still kicking five years into the marriage, then ups to you. But, if the 10-kgs-in-five-years figures are anything to go by, this celebration might be better gelato-free.

It’s his fault.

Research has shown that if one spouse gains weight, the other is likely to follow suit. Men of normal weight are 78% more likely to become obese if their wives gain weight. Similarly, women of normal weight are almost 90% more likely to become obese if their partners start collecting the kilograms.

Living together, eating together, having the same habits (excuses) when it comes to exercising are the reasons for this pattern.

This means there is a very compelling case for adopting a joint approach to healthy living. Diet together, record weights together, set each other goals, exercise together, support each other in staying healthy. And you’ll likely beat the post-marriage weight gain.

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You’re… happy?

One of the most interesting elements to the research into weight-gain and marriage is the fact couples who live together, but are not married, do not experience the same phenomena.

This, more than anything, shows the potential power of being a secure relationship. Or the way “putting a ring on it” can, quite effectively, change your mindset and – following this – your body. Maybe it’s a relaxation that comes from knowing you are “safely tied” to the other person, or the fact that you are no longer trying so hard to impress. (These are cringe-inducing cliches, but the scales don’t lie.)

It’s also about priorities. Happiness within a relationship, and security within a marriage, might mean you are no longer hitting the gym every other day on the off chance you’ll meet a cute guy at Saturday night’s party. Instead, you’re working longer hours to grow professionally, or you might be planning for – or in the midst of running – a family.

Whatever the reason for the marriage-weight-gain trend, supporting each other in healthy life choices should be a cornerstone of any relationship, and extend to decisions beyond food and exercise. This is why our focus on weight-loss during engagement, and wight-gain during marriage, should not be all-encompassing; just as there’s more to wedding planning than kale chips and juices, there’s more to a marriage than gelato and couch time.

Watch next: Twenty-Somethings on making marriage last…