9 months or 40 weeks is the normal length of pregnancy. If delivery occurs at less than 37 months then, it is a premature birth.

Apparently, all pregnant women are at risk of delivering pre-term. However, chances are even increased if there is...

• preterm labor in this pregnancy,
• history of preterm labor and delivery in a previous pregnancy,
• history of preterm ruptured membranes in a previous pregnancy,
• multiple gestation (twins, triplets, etc.),
• miscarriages between 14 and 24 weeks in two prior pregnancies,
• malformation of your uterus including fibroids,
• too much amniotic fluid in this pregnancy (polyhydramnios),
• surgery on your cervix for abnormal cells on your Pap smear,
• substance abuse, including cigarettes,
• bleeding after 14 weeks in this pregnancy,
• short cervix (less than 2 cm) at less than 24 weeks,
• false labor during pregnancy.
• 
  
  We all know that a baby's development is better, the longer they stay in the mother's uterus. Babies born prematurely do survive but there is a greater chance of death and handicaps. Hence, much is being done to lower the risk of premature delivery.
  
  However, as reported by Reuters, "a drug prescribed to pregnant women to reduce the risk of early labor could be doing patients more harm than good".
  
  

Tommy's, a charity that funds national research programs to prevent miscarriage and stillbirths, said the antibiotic metronidazole may double the risk of pre-term deliveries.

Should you (or anyone you know) be at risk, talk to your doctor and discuss your options. There surely are other ways to overcome this condition. And while you are at it, take care of yourself early on. You and your yet-to-be-born-baby will both benefit.

[additional source: Best Doctors]