One of the major risks to any pregnancy is having a preterm – also called premature – birth. This happens in about one out of ten pregnancies. The number of preterm births in the United States has been going down as pregnancy in teens is becoming less common. The danger of preterm birth depends on how early your baby is born. 
The best thing for your baby is for your pregnancy to go to 40 weeks.  Even the last few weeks of pregnancy are an important time for development of your baby’s lungs, brain, and liver. 
- Babies born before 37 weeks are preterm.
- Babies born before 32 weeks are at higher risk for long-term problems. 
- Babies born before 28 weeks are considered extremely preterm and are at the highest risk for preterm problems and may not survive. 
- Babies born before 23 weeks rarely survive. 
Common problems for babies born before 32 weeks include breathing problems, feeding problems, brain injury and damage (cerebral palsy), vision problems, and hearing problems. Very premature babies may need to be in neonatal intensive care for a long time. 
What Causes Preterm Birth?
Some women have a problem called incompetent cervix. This means that the connection between the vagina and the womb is too short. If your cervix is incompetent, the first sign may be a lost pregnancy or a preterm birth.  Cross reference for incompetent cervix https://blog.pregistry.com/incompetent-cervix/
In many cases, the cause of preterm birth is not known. There are some known risk factors that increase the chances of preterm birth. These include: 
- Teenage pregnancy
- Pregnancy after age 35
- A previous history of preterm birth
- Carrying multiple babies
- Having assisted reproductive technology (ART)
- Smoking during pregnancy
- Drinking alcohol during pregnancy
- Being under a lot of stress during pregnancy
Can Preterm Birth Be Prevented?
If you have had preterm births before or if you are known to have an incompetent cervix, you may be considered at higher risk for a preterm birth. Your doctor may consider treatment for prevention of preterm birth or treatment to protect your baby as much as possible.
- For incompetent cervix, a stich may be placed into your cervix to strengthen it. This procedure is called cerclage. The stitch is removed when you get close to your due date. 
- If you are at high risk for preterm birth, your obstetrician may prescribe progesterone. This hormone may reduce the risk of preterm birth. [1,2]
- If your baby is at high risk from preterm birth, your pregnancy care team may help protect your baby with medications. These include steroids to protect your bays lungs, magnesium sulfate to protect your baby’s brain, antibiotics to reduce the risk of infection, and medications called tocolytics, which may delay birth. [1,2]
What Can You Do?
You should talk to your pregnancy care provider about the risk of preterm birth. If you are at high risk, you should talk about options that could reduce your risk and protect your baby. You should also know the signs and symptoms of preterm birth. They are similar to labor. They include contractions, cramps, back pain, and vaginal discharge of blood or fluid. Always call your pregnancy care provider right away if you have any of these. You can also reduce your risk by: 
- Not smoking
- Not drinking or using drugs
- Getting early prenatal care and keeping all your prenatal care appointments
- Waiting about 18 months between pregnancies
CDC, Preterm Birth.
ACOG, Extremely Preterm Birth.
March of Dimes, Preterm Labor and Premature Birth.