In about half of all premature births, physicians are unable to determine what caused a woman to go into labor early. For the rest of the more than 480,000 babies born prematurely each year in the United States, there are many factors that can cause an early delivery. These include:

• Little or no prenatal care.
  
  
• Multiples. With an increasing number of women using in vitro fertilization (IVF) to become pregnant, physicians are delivering more and more sets of twins and triplets or higher multiples. The rate of twins and triplets being born prematurely is higher than for single births, with some studies showing as much as half of all twins being born prematurely.
  
  
• Previous premature birth. Women who have had a premature delivery previously have an increased risk of having another early delivery.
  
  
• Short time period between pregnancies. A period of less than six to nine months between the birth of one baby and the beginning of the next pregnancy increases the risk of premature delivery.
  
  
• Preeclampsia. Characterized by high blood pressure during pregnancy, this condition can lead to potentially serious complications for the mother and fetus including premature birth.
  
  
• Smoking. There are many well-known risks associated with smoking during pregnancy, including low birth weight babies and premature deliveries.
  
  
• Uterine or cervical abnormalities. This includes stretching or abnormally shaped uterus or cervix, as well as fibroids or even having too much (polyhydramnios) or too little (oligohydramnios) amniotic fluid (liquid that surrounds the fetus during pregnancy). Women diagnosed with an incompetent cervix may require a procedure known as a cerclage. Performed at about 12 weeks gestation, this procedure involves placing a stitch in the cervix to prevent it from opening up too soon. The stitch is then removed when the baby is closer to full term.
  
  
• Recurring infections of the urinary tract, bladder, kidney and/or vagina.
  
  
• Infection with a high fever of more than 101 degrees Fahrenheit (38 degrees Celsius) during pregnancy.
  
  
• Unexplained vaginal bleeding after the 20^th week of pregnancy.
  
  
• Chronic illnesses, such as high blood pressure, diabetes, inflammatory bowel disease (IBS), kidney disease or lupus).
  
  
• History of second-trimester abortion or more than two first-trimester abortions. The cervix may be damaged during these procedures.
  
  
• Mother’s age is younger than 18 or older than 40 years.
  
  
• Mother is underweight or overweight before pregnancy.
  
  
• Taking certain antidepressant medications (e.g., selective serotonin reuptake inhibitors) slightly increases the risk of premature birth.
  
  
• Clotting disorder (thrombophilia).
  
  
• Alcohol and drug abuse.
  
  
• Domestic violence (physical, sexual or emotional abuse).
  
  
• Low socioeconomic status, which may be accompanied by high levels of stress or poor social support.
  
  
• Trichomoniasis. Each year, more than 5 million new cases of trichomoniasis are diagnosed, making it the most common nonviral sexually transmitted disease (STD) in the United States. According to the National Institutes of Health (NIH), a woman with untreated trichomoniasis is 40 percent more likely to deliver prematurely or deliver a low birth weight child than those without the STD.
  
  
• Bacterial vaginosis (BV). This form of vaginitis involves a bacterial imbalance in the vagina that leads to an overgrowth of harmful bacteria. BV is of particular concern for women who are pregnant as this condition can lead to delivering babes who are premature or have a low birth weight.

Heredity also may play a role in the risk of premature birth, according to new research. African-American babies are more likely than babies of European descent to have a variation of the SERPINH1 gene that can lead to a reduced amount of the protein collagen. Lack of collagen can cause the fetal membranes to become weak and rupture (premature rupture of membranes), resulting in preterm delivery.