Experts believe that carefully timing a pregnancy could prevent miscarriage and boost a woman’s chances of having a baby. That is due to the number of natural killer cells – special immune cells which clear out ageing ones in the womb lining.
Some women are thought to produce too many killer cells at times, which threaten the lining with collapse. It creates a narrow window for women to get pregnant more safely, according to Professor Jan Brosens at Warwick University.
His team is offering women a test to help identify the best time to conceive and has so far advised 150 people.
Jane Brewin, chief executive of miscarriage research charity Tommy’s, said: "Professor Brosens’ work is arguably the most exciting development in miscarriage and offers real hope to those women who can get pregnant but then suffer miscarriage after miscarriage.
"The work is still experimental and not in routine clinical practice but hopefully in the next few years with more proof, things will change."
Around one in six pregnant women lose their baby, with a quarter of pregnancies ending in miscarriage within 23 weeks. But around one in 100 women lose a child repeatedly, suffering more than three miscarriages. And some suffer up to 12 miscarriages in a row, yet later have a healthy baby.
Professor Brosens told the Nordic Fertility Innovation meeting in Stockholm, Sweden, that he thinks the balance between natural killer cells and ageing cells in the womb could be to blame.
In a healthy woman, stem cells build up the womb lining by about 10mm over the 10 days following her period. This makes it thick enough for a fertilised egg to implant.
But some lining cells age, cease to divide and cause inflammation which can threaten a pregnancy. The killer cells help by clearing out ageing cells and creating a ‘honeycomb mesh’ which the embryo can implant in. But this process is believed to go wrong in miscarrying women, who gave womb samples to the Warwick team.
Researchers found that instead of having a regular number of killer cells over the course of a month, their number varied every month.
Numbers soared over several months, before crashing and then building up again. This may be due to a lack of stem cells, says Professor Brosens, whose research is reported in New Scientist magazine.
He said: "Forty percent of (recurrent miscarriage) patients had only few stem cells that could be isolated from the lining of the womb."
When more stem cells age, it is thought they attract more killer cells, which punch ever bigger holes in the lining. Ironically this makes it easier for women to get pregnant, but the large holes can make this vital structure collapse on itself.
The answer may lie in finding times when killer cells are normal, indicating that the womb is ready for a baby. Researchers have not finished the study, but are offering the test and report at least one patient who is 26 weeks pregnant.