During the British Association conference in Liverpool, Siobhan Quenby, of the University of Liverpool and the Liverpool Women's Hospital, said that tests involving 120 women had identified natural killer cells as a cause of miscarriages and failed IVF embryo implants.
Since the natural killer cells can destroy infected or malignant cells, they are beneficial in most of the body, however, in the uterus they have been found to promote rapid growth of blood vessels when present in high numbers.
The blood vessels transport additional oxygen-bearing blood and can cause miscarriages or prevent embryos implanting.
Steroids have been given to 40 women by Dr Quenby and three quarters of them have successfully given birth. All had previously had multiple miscarriages.
The treatment has been shown to reduce the level of natural killer cells in the uterus, and this is thought to increase the chances of an embryo going to full term.
"If we have higher levels of natural killer cells in the uterus we have more blood vessels and more blood flow. We've been able to find a missing piece of the jigsaw of why some women have multiple miscarriages," Times Online quoted Dr Quenby, as saying.
Steroids were first used as a potential treatment by Dr Quenby when she was looking for ideas to help Annette Quinlan, a psychiatric nurse who had suffered 19 miscarriages in 14 years.