Reasons for miscarriage
Miscarriage isn't fully understood and some miscarriages just happen without explanation other than nature 'taking a hand' and expelling an embryo that is less than perfect.
Other possibilities include:
Blighted ovum - when a fertilised egg doesn't develop as it should.
Hydatidiform mole - this isn't a real pregnancy, as there's no fertilised egg. Instead, the body responds with pregnancy hormones to the development of collection of fluid-filled sacs growing from tissue that would have become the placenta if the pregnancy had been successful. An operation may be needed to remove the mole, but hormonal treatment is also available.
Problems with the cervix - the cervix starts to open, and the uterus contracts, pushing the baby out too soon, often before the 20 weeks.
Ectopic pregnancy - the embryo is developing in one of the fallopian tubes.