Causes of pregnancy loss
There are many causes of pregnancy loss, and not all of them are well understood by medical science.
Even when all conditions are optimal, in any given month the best chance of conceiving a pregnancy that lasts long enough for you to miss a period is about 30%.
Pregnancies can be lost at anytime between fertilisation and implantation, or between implantation and full term. Pregnancy loss is much more common than many people realise. Most occur within the first 13 weeks (first trimester). Once past 12 weeks women have an estimated risk of pregnancy loss of just 2%.
About 60% of conceptions will be lost even before the period is missed, usually because of genetic abnormalities. A further 10-15% of conceptions will result in pregnancy loss. More than half of these pregnancy losses will be due to a chromosomal abnormality.
In general, about 90% of chromosomally abnormal pregnancies will miscarry in the first trimester, while about 90% of chromosomally normal pregnancies will continue to term. However, age can also play a role in pregnancy loss.