SINGAPORE - They were given only a 40 per cent chance of survival when they were born in 2004.
But the quadruplets, born premature at 26 and 28 weeks, beat the odds.
The Cheng siblings, who will turn 20 in September this year, were conceived through in-vitro fertilisation (IVF) at KK Women's and Children's Hospital (KKH) and comprise identical twins Zacharee?and Shana, and fraternal twins Abby and Yannis, the only boy in the mix.
Yannis is studying information technology at Temasek Polytechnic. His fraternal twin Abby is in business school at Nanyang Technological University.
Zacharee is in medical school overseas and hopes to become a neonatal doctor, while her identical twin Shana is waiting to start university in Australia.
Their father Cheng Ching Kang, 54, said the four were conceived through a first attempt at IVF "after eight years of not having any children".
"My wife Betty and I love kids, so we decided to try IVF. I prayed to God to let us have twins. I did not know that my wife also made the same prayer.
"I guess God decided to grant both our prayers when the doctor told us we were having four babies," said the software engineer, laughing.
Obstetricians and gynaecologists The Straits Times spoke to said such high-order multiple pregnancies are at risk of adverse outcomes, such as extreme prematurity, which can lead to chronic disease, disability and even death for the babies.
Dr Christopher Chong, who is in private practice, said the human womb is not naturally able to handle more than one or two babies without much complication.
"(There is) a higher chance of complications such as growth restriction, twin-to-twin transfusion and disproportional growth of the foetuses. Hence, selective abortion has been practised to save the other foetuses," he said.
Twin-to-twin transfusion occurs in pregnancies where twins share one placenta and a network of blood vessels that supply oxygen and nutrients essential for development in the womb.
These vessels within the placenta are not evenly distributed, resulting in an imbalance in the blood exchange between the twins.
One twin gives away more blood than it receives and runs the risk of malnourishment and organ failure, while the other receives too much blood and is susceptible to cardiac complications.
"(The option of selective abortion) would help reduce the risks involved if the procedure was successful. However, there was always an adverse outcome, including losing all the babies during the procedure," said Associate Professor Tan Lay Kok, who heads the Department of Maternal Fetal Medicine at KKH.
When that was proposed, the Chengs rejected it, wanting to keep all four babies.
The mother, Madam Betty Seah, however, was not taking to the pregnancy well due to hormonal changes, and needed constant monitoring.
She was given a series of scans that were coordinated with her antenatal clinic visits to check that the babies were growing well.
These were every four weeks from around 18 weeks into the pregnancy.
When the babies were in their 26th week, Madam Seah's water bag broke.
"Imagine my shock. I was so afraid that the foetuses might fall out, so I activated my husband, who was at work, and my brother," she told ST.
She gave birth naturally to Yannis on Sept 14 at 26 weeks, but with medication, doctors were able to help her retain the girls.
The girls were delivered by caesarean section nine days, one hour and 53 minutes later on Sept 23, putting them in the Guinness World Records that year for the longest interval between the birth of quadruplets.
This could happen only if the babies were in separate amniotic sacs.
Confirming the babies in this pregnancy were in separate "bags", Prof Tan said: "After the first baby was delivered, the contractions did not increase in intensity, and medication was given to reduce the frequency. The remaining babies remained inside the uterus, but were still at risk of being delivered sooner as the cervix was open."
The decision to keep foetuses inside the mother lies in whether they will be safer inside or out, taking into account "not just age of gestation, but also expertise at the neonatal intensive care unit (Nicu)", said Dr Chong.
"We also need to know whether it is safer for mum because multiple pregnancy itself increases the risk of high blood pressure."
Yannis almost did not make it as he was born at 26 weeks and was considered extremely preterm.
"His lungs were so underdeveloped that he needed a tube to be inserted to help him breathe. It must have been so uncomfortable as he kept pulling it out and the nurses at (KKH's) Nicu kept putting it back in," said Madam Seah, 55.
The girls were delivered by caesarean section and were also hospitalised at the Nicu as they were still premature.
Mr Cheng and Madam Seah delivered expressed breastmilk to feed the babies daily.
"There were several times when we visited and the staff were rushing around when code blue was called. It turned out to be Yannis as he had pulled out the tube and was in respiratory distress. It was the only time I saw hospital staff sprinting as if they were running a 100m race," Mr Cheng added.?
Code blue is used to describe the critical status of a patient, such as when a patient goes into cardiac arrest, has respiratory issues or experiences any other medical emergencies.
All four babies were hospitalised at the Nicu, with Yannis staying there the longest - 179 days, or almost six months.
"I remember it clearly as if it was just yesterday when the Nicu nurse asked for their names. They were known as Quad 1, Quad 2, Quad 3 and Quad 4 since birth, and we were told that we needed to register their births within 42 days from birth," Mr Cheng said.
"We had only two days left, so we flipped through the Bible to look for appropriate names. That was when we came up with Yannis, Abby, Shana and Zacharee," Madam Seah said.
Grateful to the staff at KKH for taking care of his quad squad, Mr Cheng still writes to the hospital's deputy head of the neonatology department, Dr Khoo Poh Choo, with updates and photos of the four siblings every year.
Among the four, only Zacharee was so deeply impressed by the birth stories and how her siblings and herself survived that she hopes to be a doctor one day and help other premature babies.