The later a fertilised egg implants in a woman's uterus, the more likely she is to have a miscarriage, new research has found.The report in the latest New England Journal of Medicine details the most precise information the timing of implantation - the true start of pregnancy - influences its success or failure.
Fertilised eggs can attach themselves to the lining of the womb anywhere from six to 12 days after ovulation, the researchers from the National Institute of Environmental Health Sciences found.But most successful pregnancies implant eight, nine or ten days after the egg's release from the ovary, with those implanting on day eight having the greatest chance of success.
The researchers speculate that late-implanting embryos may be weaker and that rejection of them may be a protective mechanism designed to spare the mother the physiologic burden of supporting non-viable offspring.
The new studies, which monitored the hormones in daily urine samples of healthy women trying to conceive, showed the day by day trend for the first time in a large sample (341 women). The work showed 12 per cent of pregnancies failed if implantation occurred by the ninth day, but the figure rose to 26 per cent when implantation occurred on the 10th day, 52 per cent on the 11th day and 82 per cent thereafter.The researchers found no association between late implantation and clinical miscarriages that occurred later in pregnancy.