A mom who is currently pregnant by her stepson has taken to Instagram to show off her 127kg weight loss.

The 35-year-old, from the western region of Krasnodar Krai in Russia, had previously been married to husband Alexey, 45, for 13 years before starting a new relationship with his son Vladimir, 20.

Marina Balmasheva shared news of her pregnancy on Instagram with a video where she hands Vladimir a positive pregnancy test.

Watch Vladimir's reaction to his former step-mother's pregnancy below:

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"I can't go for a walk because the sport is banned," she wrote in the post, which has been roughly translated to English, adding that being pregnant had "brought the appetite".

She then asked her female followers how much weight they gained during their pregnancies, and "how much was left at the hospital", suggesting that she's worried about getting her pre-pregnancy body back as soon as possible.

Per News.com.au, Marina said she was an "unhappy woman" when she was larger and her life's transformed now that she's a "skinny b**ch".

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After she began her weight loss journey last year, Marina was still with Vladimir's father. The lost weight left her with sagging skin, which she had surgery to remove.

She said she finally decided to lose weight four years ago when she said she realized "that this is the point".

"You do not believe yourself and in yourself either. But the head already says: Stop eating!" she wrote in another Instagram post of herself in a pool with her ex-husband.

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Now, she is happily showing off her trim physique in lingerie and swimwear.

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This comes just over a month after the 35-year-old caused outrage just over a month ago by revealing that she was in a relationship with her stepson in the first place.

Mum pregnant by stepson shows off 127kg weight loss