Doctors at Lucile Packard Children's Hospital say they will attempt to separate 2-year-old twin girls conjoined at the chest and abdomen.
It's a risky operation with a 50 percent survival rate.
Surgery on the girls from San Jose, Costa Rica, is expected to take place in late November, after their skin has been stretched to cover the large gap where they have been connected.
Since arriving in San Francisco on July 25, the girls have been receiving weekly injections of sterile salt water into balloons placed beneath their skin.
This procedure should stretch their skin to compensate for the hole surgeons will cut into their abdomens.
The girls are also connected at the right atria of their hearts, the chamber that receives blood from the rest of the body, and they share some blood and a single liver.