"I am now pregnant with his family's first grandchild. My husband and I mutually decided that we didn't want to know the sex before the birth," she wrote.
"My mother-in-law was livid with our decision, even though I tried to placate her by using the obstetrician she suggested and allowing her to attend some of my prenatal appointments. She continued to bring up gender at every opportunity."
The woman went on to say she was surprised to see her mother-in-law smiling broadly at her doctor's office one day.
"A few days later I had messages from family members congratulating me on the baby girl I was having! My mother-in-law wheedled the information out of the ultrasound technician, who is a friend of hers, then announced it."
The woman rounds off her letter by saying she felt hurt and betrayed by the actions of her husband's mother and that she and her husband had now made a decision to exclude his mother from the birth of the baby.
It's not the first time mother-in-laws have caused family angst.
In September, another anonymous UK woman on Mumsnet had the parenting network up in arms when she posted about the fact that her mother-in-law had opted for a white outfit to wear to her nuptials:
"Future MIL has bought a white outfit for my wedding," the woman, called Natalie204 on Mumsnet, wrote.
"Mil [sic] showed me a photo of her dress and jacket she has bought for our wedding it's beautiful floor length white evening dress and white jacket [sic]," she continued.
"But I was brought up to never wear just white/cream to a wedding as a guest. I mentioned that it's not good etiquette to wear all white to a wedding."
The resulting comments divided down the middle as to whether people thought this act was acceptable behaviour or not.
While some said the woman's mother-in-law should be able to dress in whatever made her feel comfortable and beautiful, others were less forgiving.
