"There are questions about how the physiological changes in the uterus will affect the mother and whether the transplanted uterus will be conducive to a growing baby," said Dr. Charles Kingsland, a spokesman for Britain's Royal College of Obstetricians and Gynaecologists and a gynecologist at Liverpool Women's Hospital.
In a study published last week, Brannstrom and colleagues described the procedures used to transplant the nine wombs and said there were "mild rejection episodes" in four patients.
He said the transplanted wombs would be removed after a maximum of two pregnancies.
Other experts called it a promising step but said it would be crucial that babies get enough nutrients from the mother's blood supply.
"We really don't know if the blood flow to the uterus will increase and adapt in the same way," as in a regular pregnancy, said Dr. Yacoub Khalaf, director of the Assisted Conception unit at Guy's and St. Thomas' hospital in London.
"It is a good sign they have done the (embryo) transfers," Khalaf said. "But a live birth will be the best validation that this works."