What To Expect states that while there is no known definitive answer to why a woman gives birth or goes into labor early, doctors point to several possible reasons premature labor occurs. These include infection, gestational diabetes, preeclampsia, and abnormalities of the uterus or cervix. Certain risk factors increase a woman's chances of giving birth early, according to WebMD.
Premature labor refers to birth that occurs earlier than three weeks before the baby's due date. What To Expect reports that nearly half of all early births are the result of a uterine or vaginal infection. Specifically, bacterial vaginitis and untreated urinary tract infections are known to cause preterm labor to occur. Women who suffer from pregnancy complications, such as gestational diabetes and preeclampsia, are at a higher risk for giving birth early. Carrying a baby full term is difficult if the uterus or cervix is damaged or misshapen; many women with structural abnormalities are unable to carry a baby to a full 40 weeks.
Stress and other outside factors also play a role in premature labor. Extreme stress or emotional distress can cause the body to go into labor, according to What to Expect. WebMD lists smoking, drinking alcohol, being overweight or underweight, or various pre-existing conditions as the most common risk factors for premature labor.