It is a decision no parent wants to make.
But US couple Nicole and Christian McDonald were faced with the ultimate dilemma when it came to their 13-month-old conjoined twins, Anias and Jadon.
Should they agree to incredibly rare and delicate surgery to separate them, even if it meant risking losing one or both of their sons in the process?
Conjoined at the head, the boys are known as caniopagus twins, a rare phenomenon that occurs in just one in every 2.5 million births.
But the brave couple put their trust in the medical team at Bronx's Montefiore Medical Centre, a place Nicole called the 'best second home we've ever had'.
While the family were originally hesitant to go public with their story, they wanted the chance to thank the incredible doctors and nurses who were caring for their boys, calling them the 'true heroes of our story'.
News of the operation soon made headlines around the world, with support flooding in for the couple and their eldest son, Aza.
Incredibly, after a marathon 27-hour surgery, the boys were separated and ready to begin lives as individual twins.
The separation procedure itself lasted 16 hours, followed by more hours to carefully rebuild their skulls.
Jadon was the first twin to be wheeled into recovery.
But Nicole said it was difficult to comprehend her boys being apart for good.
'When they told me they were wheeling Jadon up first, it took me a second to comprehend. I actually asked why they rearranged the room because I hadn't really internalized the idea that there would be two beds in here. Welcome back my sweet Jadon. Happy rebirth day,' she wrote on her Facebook page.
More than five hours later, Anias was reunited with his brother after some complications.
Updating her followers, Nicole then wrote an emotional tribute to Anias.
'You look so small in that big bed all by yourself. I know things got pretty hard...and that you've been through more than I will ever experience in a lifetime...but I promise that it's going to be ok,' she wrote.
But Nicole explained it will still be some time before the full results of the operation are known.
'The overall atmosphere was one of celebration mixed with uncertainty,' she said.
'Anias really got rocked in this procedure. It really now is up to God in terms of how he recovers.'
The complex operation cost around $2.5 million.
To donate to the family, visit their GoFundMe page.