Courtney Williams Baker shared a photo of her 15-month-old daughter Emmy holding the letter that was being posted to her prenatal specialist.
"[The prenatal specialist] repeatedly suggested we abort," Baker claimed in the Facebook status.
"He said her and our quality of life would be horrible. He was so unbelievably wrong."
Baker said she wanted to do something to "advocate" for children with Down's syndrome, so decided to highlight why she's grateful for her daughter Emmy.
She also referenced a friend, who has a son with Down's syndrome, who had a much better experience with their prenatal specialist.
"Her prenatal specialist would see her child during her sonograms, he would comment: 'He’s perfect'," Baker wrote.
"Once her son was born with Down's syndrome, she visited that same doctor. He looked at her little boy and said: 'I told you. He’s perfect'."
Baker shared the letter on Parker Myles Facebook page - a page run by a mum who has a child with Down's syndrome and has nearly 19,000 followers.
Baker wrote in the letter: "While I was so grateful for my friend’s experience, it filled me with such sorrow because of what I should have had.
"I wish you would have been that doctor.
"I came to you during the most difficult time in my life. I was terrified, anxious and in complete despair. I didn’t know the truth yet about my baby, and that’s what I desperately needed from you.
"But instead of support and encouragement, you suggested we terminate our child. I told you her name, and you asked us again if we understood how low our quality of life would be with a child with Down's syndrome.
"You suggested we reconsider our decision to continue the pregnancy."
Baker went on to say she "dreaded" her appointments and although it doesn't make her angry, it makes her sad when she looks back on it.
"I’m sad you were so very wrong to say a baby with Down's syndrome would decrease our quality of life," she continued.
"And I’m heartbroken you might have said that to a mummy even today. But I’m mostly sad you’ll never have the privilege of knowing my daughter, Emersyn.
"Because, you see, Emersyn has not only added to our quality of life, she’s touched the hearts of thousands. She’s given us a purpose and a joy that is impossible to express.
"She’s opened our eyes to true beauty and pure love."
Baker ended the letter by saying she prays no other mother has to go through what she did.
"And my prayer is when you see that next baby with Down's syndrome lovingly tucked in her mother’s womb, you will look at that mummy and see me then tell her the truth: 'Your child is absolutely perfect'.”
A spokesperson from the Down's Syndrome Association (DSA) told The Huffington Post UK they run ‘Tell It Right, Start It Right’ (TIR) Royal College of Midwife accredited study days to address this very issue.
"Our key objective is to ensure that health professionals who work in antenatal, neonatal and postnatal care must provide expectant parents with accurate, up-to-date and balanced information about living with Down’s syndrome today," they said.
"They must include correct information about the life prospects of people with Down’s syndrome, the impact on families, support available in the community and the joys and challenges of having a child with Down’s syndrome.
"The TIR training assists them to support expectant parents through the screening process by sharing information in a non-directive manner.
"The DSA expect respect and support for parents making choices about antenatal tests and their outcomes – whatever they decide to do."
The DSA suggests expectant parents gather as much information as possible about the "joys and challenges of having a child with Down’s syndrome".
"We encourage expectant parents to call our information officers on 0333 1212 300 and to look at the wealth of information available on our website," they added.
The Facebook post, uploaded on 5 June, had more than 3,000 shares in three days.