The rare condition is caused by a combination of four heart defects. The unborn baby will have to have many open heart surgeries and quality of life will be impacted.

Katie Busby and partner Mathew Yalouris were shocked to their bones when doctors informed them of their unborn baby's diagnosis.

"It was horrible to hear, it's very upsetting," Katie told the Argus.

"It has put a lot of stress on the family and I am incredibly upset but I am trying to keep it together."

The devastating news came two days after the couple had decided to name their first daughter together Thea, after Mathew's father Theodore.

The father and son have an incredible bond and are "two peas in a pod".

Katie told the Argus the pregnancy was "normal" until she went for a regular morphology scan at the 18-week mark.

"Thea's heart measurements weren't readable so we scheduled in another test for two weeks later but they were sill unable to get a reading, which raised alarms," Katie said.

"We went to John Hunter Hospital on May 2, where further tests were run and we learned of the diagnosis after meeting with a cardiologist expert from the Maternal Fetal Medical unit."

The irregularities found affect the structure of the heart, which causes oxygen-poor blood to flow out of the heart and to the rest of the body.

Depending on the state of Thea when she is born, she will have to have open heart surgery either directly after birth or within the first three months.

The best case scenario is Thea will be born with a pink tinge to her skin and doctors will run a series of tests, which will advise the doctors whether Thea's heart is pumping enough blood to allow her the time to feed and grow stronger before open heart surgery.

However, a blue tinge to her skin will suggest the oxygenated blood is not flowing as well and open heart surgery will potentially be required immediately.

Untreated cases of TOF usually lead to severe complications over time and it's expected Thea will need many surgeries throughout her life.

"I am terrified she will come out blue, then she will need to have surgery straight away and anything can happen," Katie said.

"The first surgery will have implications on her forever; how her quality of life is, the amount of surgeries she will be required to have and how soon the next surgery will be."

TOF isn't curable, as it is a congenital heart defect. The surgeries will improve Thea's standard of life but the problem cannot be completely fixed.

There is nothing that can be done to help Thea until after her birth and for Katie, that's the most unbearable feeling.

"The doctors told me 'there is nothing you can do'. It's hard as a mother to sit back and do nothing," she said.

"It's frustrating just sitting here waiting to see what happens, hoping, that there's no drastic turns for the worst."

Katie will continue to travel fortnightly to John Hunter Hospital to receive check ups but as the due date nears, the cardiology team may determine more regular tests or a move to Westmead in Sydney is required.

Both Katie and Mathew are hopeful the birth can take place at the John Hunter Hospital for the logistical factor. The hospital is closer to the family home and it will make it possible for the children to visit their mother.

As Katie continues to come to terms with Thea's heart condition, she has made the difficult decision to wait until after the birth to prepare her bedroom.

"I refuse to set up her cot, there's still that risk that she may not make it," Katie said.

"I don't want to come home empty handed and have a room set up, it would be devastating."

Katie's concerns only begin with Thea, as the blended family have four children and have custody of three full-time.

The 27-year-old met Mathew when they worked together at Nestle. Since starting their relationship, Katie has taken on the role of full-time mum as she cares for Holly, 6, Sophia 4, and Liam 3. Mathew's eldest daughter Gracie, 10, resides with her biological mother.

Mathew, who is a casual employee, isn't eligible for paid leave and the family will struggle to afford for the 34-year-old to have time off work for the birth and surgeries of Thea.

Katie has growing concerns for their other children, especially Sophia and Liam, who have recently been diagnosed with autism.

"I am scared about how this will impact the other children as well, they are at a young age and need their parents around," Katie said.

"The two youngest have just been diagnosed at level two on the autism spectrum disorder and they need to stick to a strict routine. It's already been tough on them and I am concerned at how we will cope."

In an attempt to be better prepared financially, the family has been raising funds in anyway possible.

Katie became committed to the return and earn recycling scheme and family and friends are beginning to chip in too.

"Since the diagnosis, family and friends have been supporting us in anyway possible," Katie said.

"I started the return and earn scheme to get every possible dollar we could and they have started to donate their used bottles and cans to help."

Nestle Smithtown have generously donated hampers and the South West Rocks Country Club, Mathew's former employer, have extended the offer to help the family with fundraising.

The family also held a barbecue at the Kempsey Woolworths and raised $400 last Saturday.

Thea's scheduled birth date is on September 9 and all of our fingers and toes are crossed for the best possible outcome.

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