Four women who received womb transplants have had embryos transferred into them in an effort to become pregnant, according to a Swedish doctor. It has not been disclosed if any pregnancies have actually occurred.

Nine women have received wombs donated by their mothers or other close relatives as part of such attempts since 2012. The experimental procedure is meant to test the possibility of transferring a uterus so a woman can give birth to her own biological child. The women had in-vitro fertilization prior to the transplants, using their own eggs for the creation of embryos.

"We have already begun transferring embryos into four of the women and plan to make attempts with the others when they are ready," said Dr. Mats Brannstrom, professor of obstetrics and gynecology at the University of Goteburg.

Brannstrom, who is leading the research, predicted that three or four of the women might successfully have a child.

"One or two more will perhaps get pregnant and miscarry and one or two won't be able to get pregnant," he said.

In two previous attempts to transplant a womb, in Turkey and Saudi Arabia, a baby was not produced. Doctors in Britain and Hungary are planning similar procedures but using wombs from deceased donors. This marks a point of controversy with Brannstrom, who uses wombs from living donors.

In a study published last week, Brannstrom's team said that "mild rejection episodes" had taken place in four women. Two patients required a uterus removal due to compliations.

Dr. Richard Smith, head of the UK charity Womb Transplant UK, said that large chunks of blood vessels had to be removed from the donor along with the uterus itself, increasing the risk of the operation.

Aside from being watched carefully, the women will have to take drugs throughout their pregnancy to makes sure their bodies do not reject the organ.

"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, spokesman for Britain's Royal College of Obstetricians and Gynecologists.

According to Brannstrom, the transplanted wombs would be removed following a maximum of two pregnancies.

Some see the procedure as a promising step but stress the importance of babies receiving 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," 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," he added. "But a live birth will be the best validation that this works."