'We are praying and trusting in God that he will help us.'
Ms Casanellas is convinced that her obstetrician, Dr Alejandro Guidos who is currently under arrest, had planned the swap all along.
Mercy, a missionary working in a large church in San Salvador, has been based in Dallas, Texas with her British husband Richard Cushworth.
The Cushworths' son was due to be born on June 20th, and in order to be present at the birth, Mr Cushworth had departed for a month-long trip mid-May.
However, just a few days later - five weeks ahead of due date - Mercy was told by Dr. Guidos that she would need an emergency Cesarean.
Changeling: Ms Casanellas, pictured with the couple's biological son after the birth, says that she noticed that the child given to her by hospital staff in San Salvador was darker and looked nothing like her or her husband
Horror: Mr Cushworth and Ms Casanellas, pictured with Jacob, told MailOnline that she wanted to believe that Jacob was their biological son, but that she felt she needed to do a DNA test in the end
'Richard was out of the country and suddenly the doctor said 'no the baby has to be delivered today' so he got me to the hospital immediately and got the baby out by c-section.
'The anesthesiologist, the pediatrician - the entire team was chosen by him, I did not know them.'
Speaking to MailOnline on Monday, Ms Casanellas also revealed that Dr Guidos had been unusually close to her during her pregnancy.
'One night in January we feared that I was about to have a miscarriage, and he stayed with me the whole night, stayed by my bedside and cared for me.
'I considered that very nice of him, very professional, and it was not until I told the investigators about this after the swap that they told me that this is not how it is done.
'He was also the only person who knew that my husband was British and that he would be out of the country for one month.'
Ms Casanellas was taken to the Centro Ginecologico, a private hospital in San Salvador, and underwent an emergency Caesarean on May 21st this year, at 35 weeks pregnant.
Under arrest: Ms Casanellas say she is convinced Dr Alejandro Guidos, who was arrested on Thursday, orchestrated the baby swap, and that he was unusually attentive to her during her pregnancy
'I saw my son at 9.30pm when he was born, and then they put me to sleep. In the morning, around 8am, they brought in everyone's baby, but they never brought mine.
'I told them 'Where is my baby? Where is my son?' but they just told me: 'He is coming, he is coming.'
'They brought him at lunch time, and I asked them 'why is this baby different?' He has darker skin, his facial features were different, his nose, his eyebrows.
'They told me that it was because he was a newborn and that he was swollen and that he would change.
'They said his skin would get lighter and that the swelling would go down. They said everything was ok, so I believed them.
'But then my family started to make comments, they kept saying 'this baby has darker skin', and 'he doesn't look like your husband and he doesn't look like you' for the next two months.'
After Mercy had recovered from her c-section she and her husband began readying themselves for their return to Dallas, Texas, where she is set to study for a Masters degree.
'I didn't want to think about the idea of a switch because I am very connected to my baby and I breastfed him every day - I still do.
'I thought, 'how could I even think that this is not my child?', but the thoughts were always on my mind, and finally I made the decision to to a DNA test because I didn't want to have these thoughts hanging over me as I went to study for my Masters.'
Unbelievable: After Ms Casanellas gave birth, she was given an anesthetic and when she woke up, she claims she was given a different baby (pictured is their biological son, taken by a family friend)
Not theirs: Ms Casanellas and Mr Cushworth fear their light-skinned baby (pictured) was snatched deliberately by staff at the hospital, in order to sell the boy to child traffickers
The DNA test, which was sent from El Salvador to two separate labs in the U.S., came back to show that there is zero per cent chance of Mercy and Richard being the biological parents of little Jacob.
Ms Casanellas and Mr Cushworth, whose parents hail from Bradford, West Yorkshire, now fear their light-skinned baby was snatched deliberately by staff at the Ginecologico private hospital, in order to sell the baby boy to child traffickers.
In an emotional interview with a local TV station, a teary Ms Casanellas said: 'We haven't been able to sleep thinking about where he is, and who has him.
'We just want them to give us our son back.' Mr Cushworth, who met Ms Casanellas when he worked as a missionary in El Salvador, added: 'It's a horrible situation. I have a child and I don't know where he is.
Ten babies born on the same day, including the Cushworths son: four boys and six girls.
All the four boys have had DNA tests, which all parents agreed to - they expect the results of this on Tuesday or Wednesday.
Mercy and the Cushworth family also have further proof that their child may even have been swapped for a baby from a different hospital.
In the pictures taken by Mercy's friend who was present at the birth, their biological child can be seen with a yellow clip around his umbilical cord - a clip which is not removed until the cord falls off naturally.
However, the baby that was handed back to Mercy had a white clip around his umbilical cord, one which the family believes is from a different hospital.
'We had the footprints taken of my child when he was born for his birth certificate. Later when I went back to change a mistake on the document I noticed that the prints on his certificate were different to the ones we had - they were smaller and a different pattern.'
At first San Salvador's Ginecologio hospital, considered the best private hospital in the country, denied that the baby could have been swapped, saying it was 'impossible' due to their 'high standards' of control.
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