Here's another one for the "mother-in-law from hell" files: a woman has revealed the cunning way she found her mother-in-law (MIL) used to mess with her birth control.

Under the pretense of helping out with dinner duties, the MIL managed to tamper with the couple's contraceptives.

The woman took to Reddit to share her story, explaining that both her and her husband are "adamantly childfree".

"For those of you who don't know exactly what that means, basically we don't want to have kids ... ever. We are in our late 20s and we enjoy our carefree lives so much that we decided that we thought it would be best to never have kids," she wrote.

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She recalls the day the couple explained their position to the MIL and she "went into hysterics", worried her "bloodline and family name will die".

Needless to say, Christmas was a little awkward that year.

And then it got worse.

One night, the MIL offered to help with dinner. She realised there were some ingredients missing and, while the woman was still at work, the MIL sent her son to the shops for the forgotten items.

While her husband was out, the Reddit user arrived home from work and went straight to her bedroom to get changed.

"When I opened the door into our bedroom what I see is what looks like MIL carefully trying to poke holes into our condoms," she wrote.

"She snaps her head in my direction with this mortifying deer in the headlights look on her face. Like a fish out of water.

"She mumbles something about how she's 'trying to save the family' and frantically runs out of the house before I even have a chance to say anything."

The MIL got in the car and disappeared down the road.

The woman shared that her husband was "absolutely horrified" when he got home and was told of what had happened.

People on Reddit tried to advise the woman on what to do about her MIL, with some considering she pressed charges.

"On a practical level I would chuck every single condom you currently have and buy new ones, if you're crazy enough to let her back into your house make sure they are somewhere she can't access," one person said.

"I would also take a pregnancy test to be sure, if she tampered with your BC it's better to know now if it has caused a problem.

"I would try and get proof of her admitting to the tampering (emails or texts etc) and if heaven forbid you are pregnant I would seriously consider pressing charges against her."

In a new post on Reddit, the woman later announced she had, indeed, fallen pregnant.

"I'm so distraught that I haven't been able to sleep in the last four days. I'm sorry I didn't update sooner, but I've been in a really dark place," she wrote.

"I can't believe I'm in this situation right now. I don't know what I'm going to do."