Some women may be letting even poor quality embryos implant
"Super-fertility" may explain why some women have multiple miscarriages, according to a team of doctors.
They say the wombs of some women are too good at letting embryos implant, even those of poor quality which should be rejected.
The UK-Dutch study published in the journal PLoS ONE said the resulting pregnancies would then fail, reports the BBC.
One expert welcomed the findings and hoped a test could be developed for identifying the condition in women.
Recurrent miscarriages - losing three or more pregnancies in a row - affect one in 100 women in the UK.
Doctors at Princess Anne Hospital in Southampton and the University Medical Center Utrecht, took samples from the wombs of six women who had normal fertility and six who had had recurrent miscarriages.
High or low-quality embryos were placed in a channel created between two strips of the womb cells.
Cells from women with normal fertility started to grow and reach out towards the high-quality embryos. Poor-quality embryos were ignored.
However, the cells of women who had recurrent miscarriages started to grow towards both kinds of embryo.
Prof Nick Macklon, a consultant at the Princess Anne Hospital, said: "Many affected women feel guilty that they are simply rejecting their pregnancy.
"But we have discovered it may not be because they cannot carry, [but] it is because they may simply be super-fertile, as they allow embryos which would normally not survive to implant."
He added: "When poorer embryos are allowed to implant, they may last long enough in cases of recurrent miscarriage to give a positive pregnancy test."
This theory still needs further testing and will not explain all miscarriages.
Dr Siobhan Quenby, from the Royal College Obstetricians and Gynaecologists, told the BBC: "This theory is really quite attractive. It is lovely. It's a really important paper that will change the way we think about implantation."
"It had been thought that rejecting normal embryos resulted in miscarriage, but what explains the clinical syndrome is that everything is being let in."
She said research would now need to discover whether this could be tested for in women and whether their receptiveness to embryos could be altered.