Miscarriages happen because the baby is not growing properly in the first few weeks of life, according to new research.
The study concludes that the crown rump length (CRL) of the fetus was "significantly smaller" in pregnancies that subsequently ended in miscarriage.
Dr Faizah Mukri, from the early pregnancy unit at St George's Hospital in London, led the study and published her findings in BJOG: An International Journal of Obstetrics and Gynaecology. While 70% of babies delivered normally had a smaller than expected CRL, she said, all those involved in miscarriage had a smaller CRL, and 61% of those had a far smaller CRL.
The study involved women who knew the exact date of their last period receiving an ultrasound scan between five and 10 weeks gestation.
Dr Mukri said the greater the difference between the observed and expected CRL at the time of the scan, the greater the risk of subsequent pregnancy failure before 12 weeks.
She said in an "ideal world" women might be offered a scan before 12 weeks to check the CRL. But few NHS hospitals have an early pregnancy unit like the one at St George's, where women can walk in and get checked if they wanted reassurance.