What are preterm labor and preterm birth?

If you start having regular contractions that cause your cervix to begin to open (dilate) or thin out (efface) before you reach 37 weeks of pregnancy, you're in preterm labor, also known as premature labor. If you deliver your baby before 37 weeks, it's called a preterm birth and your baby is considered premature.

About a quarter of all preterm births are intentional. For example, your medical team might decide to induce labor early or perform a cesarean section if you have a serious medical condition such as severe or worsening preeclampsia or if your baby has stopped growing.

The rest are known as spontaneous preterm births. You may end up having a spontaneous preterm birth if you go into labor prematurely, if your water breaks early (called preterm premature rupture of the membranes or PPROM), or if your cervix dilates prematurely with no contractions (called cervical insufficiency).

What are the symptoms of preterm labor?
Call your doctor right away if you're having any of the following symptoms in your second or third trimester (before 37 weeks):

• An increase in vaginal discharge

• A change in the type of discharge — if it becomes watery, mucus-like, or bloody (even if it's pink or just tinged with blood)

• Any vaginal bleeding or spotting

• Abdominal pain, menstrual-like cramping, or more than four contractions in one hour (even if they don't hurt)

• An increase in pressure in the pelvic area (a feeling that your baby is pushing down)

• Low back pain, especially if you didn't previously have back pain
  
  These symptoms can be confusing because some of them, such as pelvic pressure or low back pain, occur during normal pregnancies, too, and sporadic early contractions may just be Braxton Hicks contractions. But it's always better to be safe than sorry, so call your midwife or doctor right away if you're experiencing anything unusual.
   

What causes spontaneous preterm birth?

Although the cause is often unknown, a variety of factors play a role in preterm birth:

• Certain genital tract infections, such as chlamydia, bacterial vaginosis (BV), and trichomoniasis, are associated with preterm delivery. Substances produced by bacteria can weaken the membranes around the amniotic sac and cause it to rupture early. Even when the membranes remain intact, bacteria can cause preterm labor if they get into the amniotic fluid or sac. You may have been checked for chlamydia and gonorrhea at your first prenatal visit, and you would have been treated immediately if you'd tested positive for either of these sexually transmitted infections. If you've had a previous preterm birth, you may also have been screened for bacterial vaginosis. Although some studies show that treating BV in the second and third trimesters reduces the risk of preterm labor in women with a history of preterm birth, other research has found that it makes no difference. So experts don't agree on whether it's worthwhile to test pregnant women who don't have symptoms. (If you have symptoms of bacterial vaginosis, you'll be tested and treated with antibiotics, if needed.) You probably won't be tested for trichomoniasis unless you have symptoms. Some research suggests that treating women for trichomoniasis during pregnancy actually increases the risk of preterm birth.

• Having a problem with the placenta, such as placenta previa or placental abruption.

• Having structural abnormalities of the uterus or cervix, such as a cervix that's shorter than 25 millimeters and that effaces or dilates without contractions (cervical insufficiency).

• Having an excessively large uterus, which is often the case when you're pregnant with multiples or have too much amniotic fluid.

• Certain chronic maternal illnesses may be related to preterm labor, such as diabetes, sickle cell anemia, severe asthma, lupus, inflammatory bowel disease, and chronic active hepatitis. Other conditions to watch for include non-uterine infections, such as a kidney infection or pneumonia; abdominal surgery, such as having your appendix taken out; trauma to the abdomen; and periodontitis (a gum infection that goes into the bone and other tissues that support your teeth).