The Causes of Preterm Labor

Pregnant Caucasian woman holding her belly in hospital
JGI/Jamie Grill/Blend Images/Getty Images

In the United States about 12% of all babies, about 1-in-8, are born prior to 37 weeks gestation. Preterm birth is the leading cause of death in a newborn. It is also the leading cause of death for infants in many states. Unfortunately the trend is expected to get worse.

We are still trying to understand why preterm labor happens, causing babies to be born before their time. However, currently we only know what has caused the preterm labor in about half of all of the labors being effected.

When preterm labor starts on its own (as opposed to being induced), it is most commonly caused by:


About 40-50% of all preterm labor can be traced to infection. Many women do not show classic signs of infection, like fevers. One good note here is that with antibiotic treatment for Group B Strep, we have seen a reduction in the rates of preterm birth. Other than for Group B Strep, antibiotics usually lead only to antibiotic resistance and not a reduction in preterm labor rates.


There are many reasons that you might have bleeding in pregnancy like a subchorionic hemorrhage, placenta previa, etc. It also includes bleeding disorders that may be genetic or acquired. This does include placental abruption, where the placenta tears away from the uterine wall too early. Bleeding of any sort should be reported to your doctor or midwife.

Stretching of the uterus. 

The stretching or over distension of the uterus has also been linked to preterm labor and birth.

This can be caused by fibroids, multiple pregnancies (twins, triplets, etc.), or even having too much amniotic fluid (polyhydramnios). This is something that your doctor will have told you is a risk factor before it happens and the situation will likely be monitored during your prenatal care.

About 25% of all preterm births to mothers with one baby are ended early for maternal or fetal well-being.

This can be achieved either through induction of labor or by planned cesarean section, depending on the reason for the early arrival and the condition of mom and baby. The American College of Ob/GYNs (ACOG) says that there should not be any induction done prior to 39 weeks without medical reason (often called social induction), because of the increased risks. That leaves 75% of preterm labors as spontaneous. About a quarter of the spontaneous preterm labors are due to premature rupture of membranes (PPROM).

Research is ongoing as to the causes of preterm labor and how to prevent this dangerous complication. The biggest thing to be aware of are the signs of preterm labor and what risk you are at with your current pregnancy in terms of risk. If you have any signs of contractions, labor or bleeding, you should talk to your doctor or midwife right away. If you can't get ahold of them, you should consider going to the emergency room (ER). A quick diagnosis often means better outcomes for your and your baby.


ACOG practice bulletin no. 127: Management of preterm labor. Obstet Gynecol. 2012 Jun;119(6):1308-17. doi: 10.1097/AOG.0b013e31825af2f0. American College of Obstetricians and Gynecologists; Committee on Practice Bulletins—Obstetrics.

Gabbe SG , Niebyl JR, Simpson JL, Galan H, Goetzl L, Jauniaux ER, Landon M. (2012). Obstetrics: Normal and Problem Pregnancies (6th ed.): Churchill Livingstone.

Continue Reading