Surviving Shrewsbury conjoined twin Faith Williams has had a successful operation to close her chest following heart surgery last week, Great Ormond Street Hospital said.
Faith, who was born on November 26, has already survived surgery to separate her from twin sister Hope and remains in a stable but critical condition, the hospital said.
Hope failed to survive the initial operation because her lungs were too small to support her breathing.
Faith's mother, 18-year-old Laura Williams, is Britain's youngest mother to give birth to conjoined twins. Her husband, Aled, is the twins' father.
A GOSH spokeswoman said: "Faith Williams has had her chest closed at Great Ormond Street Hospital, following her heart surgery last week.
"Faith has tolerated this procedure, and remains stable but critical in the intensive care unit. Laura and Aled would like to thank the staff who performed the procedure, and those who continue to offer Faith the highest possible levels of care."
The twins were joined from the breastbone to the top of the navel and had a shared liver but separate hearts.