Smoking during pregnancy doesn't help, and can cause lightweight babies who are born premature. Sometimes there might be a complication which means labor has to be induced or labor is triggered...
If premature babies are born in hospitals they are placed in incubators where they may eventually grow to normal size and be released to their mother's care, but sometimes a very premature baby...
Pre = before
It is early
Pre-mature babies (that are born earlier than normal) are much smaller and weigh less than those who are born on time, or late.