Nearly always the cause is never identified. But some recognised causes of premature birth include:
Foetal abnormality (there is something abnomal about the baby), multiple pregnancy (sometimes when there are twins or triplets they are born earlier), extremes of maternal age (i.e. age of your mum – whether really young or really old), serious infection or maternal urinary tract infection.
Most premature babies can survive these days because of the excellent care they receive in hospitals in the UK!