Thursday, June 10, 1999 Published at 10:42 GMT 11:42 UK
Miscarriage risk of slow eggs
Once fertilised, the egg attaches to the uterus
The later a fertilised human egg attaches to the uterus, the less likely it is to survive, researchers have said.
That finding meant that, along with miscarriages later in pregnancy, one third of all embyos failed.
Now they have found what sort of embryos fail - and it is all down to timing.
Risk doubles each day
Eggs are usually fertilised in the fallopian tubes and then travel to the uterus, where they embed themselves to grow.
Those that implanted nine days after fertilisation had a 13% chance of being miscarried, the scientists found.
But if implantation occurred on the tenth day, the risk increased to 26%. On the eleventh day, it rose to 52%.
Any later and the risk of miscarriage was 82%.
However, the researchers could not explain why eggs that attach later are more likely to miscarry.
Publishing their findings in the New England Journal of Medicine, they suggested eggs that take a long time to implant are defective in some way, and would be unlikely to last a full pregnancy.
"The uterus may be receptive to pregnancy only during a limited time-window, shutting out defective embryos that get there too late," they said.
"This would spare a mother the physiologic burden of supporting a non-viable embryo."
Professor Ian Craft, head of the London Fertilty Centre, said the finding raised some interesting possibilities, and he was sympathetic towards the researchers theory.
"The transit time from where it is conceived to the uterus may well be much slower in abnormal embryos.
"It raises some interesting physiological questions - is there a dialogue going on between the egg and the tube, and how fast it moves down the tube?"
He said little was known about some of these reproductive mechanisms, but the thought of the egg communicating with the uterus was fascinating.
The researchers said their study - of 200 women in North Carolina - was relatively large. They monitored the women by measuring hormone levels in daily samples of urine.