Got these examples from webMD:
Smoking
Being very overweight or underweight before pregnancy
Not getting good prenatal care
Drinking alcohol or using street drugs during pregnancy
Having health conditions, such as high blood pressure, preeclampsia, diabetes, blood clotting disorders, or infections
Being pregnant with a baby that has certain birth defects
Being pregnant with a baby from in vitro fertilization
Being pregnant with twins or other multiples
A family or personal history of premature labor
Getting pregnant too soon after having a baby
Stress, medical reasons, mother was a premie herself, baby was ready. That's all I've been told!
I'm sorry I was just kind of "bumping" this as I was also curious, so it stayed up in the feed so others would maybe answer soon