Preterm labor may be triggered by
Another ascending uterine infection (commonly due to group B streptococci)
Fetal or placental abnormalities
Uterine abnormalities
Pyelonephritis
Some sexually transmitted diseases (STDs)
A cause may not be evident.
Prior preterm delivery and cervical incompetence Cervical Insufficiency Cervical insufficiency (formerly called cervical incompetence) is painless cervical dilation resulting in delivery of a live fetus during the 2nd trimester. Transvaginal cervical ultrasonography... read more 
increase the risk.
Premature labor can increase risk of intraventricular hemorrhage Intracranial Hemorrhage The forces of labor and delivery occasionally cause physical injury to the infant. The incidence of neonatal injury resulting from difficult or traumatic deliveries is decreasing due to increasing... read more 
in neonates; intraventricular hemorrhage may result in neurodevelopmental disability (eg, cerebral palsy Cerebral Palsy (CP) Syndromes Cerebral palsy refers to nonprogressive syndromes characterized by impaired voluntary movement or posture and resulting from prenatal developmental malformations or perinatal or postnatal central... read more ).
Diagnosis
Clinical evaluation
Diagnosis of preterm labor is based on signs of labor and length of the pregnancy.
Anovaginal cultures for group B streptococci are done, and prophylaxis is appropriately initiated. Urinalysis and urine culture are done to check for cystitis and pyelonephritis. Cervical cultures are done to check for STDs if suggested by clinical findings.
Most women with a presumptive diagnosis of preterm labor do not progress to delivery.
Treatment
Antibiotics for group B streptococci, pending anovaginal culture results
Tocolytics
Sometimes corticosteroids (eg, between 24 and 34 weeks)
Consideration of progestin in future pregnancies
Bed rest and hydration are commonly used initially.
Management of preterm labor may also include antibiotics, tocolytics, and corticosteroids (1 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ).
Antibiotics
Antibiotics effective against group B streptococci are given pending negative anovaginal cultures (2 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ). Choices for antibiotics include the following:
For women without penicillin allergy: Penicillin G 5 million units IV followed by 2.5 million units every 4 hours or ampicillin 2 g IV followed by 1 g every 4 hours
For women with penicillin allergy but a low risk of anaphylaxis (eg, maculopapular rash with prior use): Cefazolin 2 g IV followed by 1 g every 8 hours
For women with penicillin allergy and an increased risk of anaphylaxis (eg, bronchospasm, angioneurotic edema, or hypotension with prior use, particularly within 30 minutes of exposure): Clindamycin 900 mg IV every 8 hours if anovaginal cultures show susceptibility; if cultures document resistance or results are unavailable, vancomycin 20 mg/kg IV every 8 hours (maximum dose of 2 g)
Tocolytics
If the cervix dilates, tocolytics (drugs that stop uterine contractions) can usually delay labor for at least 48 hours so that corticosteroids can be given to reduce risks to the fetus. Tocolytics include
Magnesium sulfate
A calcium channel blocker
Prostaglandin inhibitors
No tocolytic is clearly the first-line choice; choice should be individualized to minimize adverse effects.
Magnesium sulfate Magnesium sulfate Preeclampsia is new-onset or worsening of existing hypertension with proteinuria after 20 weeks gestation. Eclampsia is unexplained generalized seizures in patients with preeclampsia. Diagnosis... read more is commonly used and is typically well-tolerated.
IV magnesium sulfate should be considered in pregnancies < 32 weeks. In utero exposure to the drug appears to reduce the risk of severe neurologic dysfunction (eg, due to intraventricular hemorrhage), including cerebral palsy, in neonates.
Prostaglandin inhibitors may cause transient oligohydramnios. They are contraindicated after 32 weeks gestation because they may cause premature narrowing or closure of the ductus arteriosus.
Corticosteroids
If the fetus is ≥ 24 weeks and < 34 weeks, women are given corticosteroids unless delivery is imminent. Another course of corticosteroids can be considered if all of the following are present:
The pregnancy is < 34 weeks.
Women are at risk of delivering within 7 days.
The last course was given ≥ 14 days prior (3 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more , 4 Treatment reference Prelabor rupture of membranes is leakage of amniotic fluid before onset of labor. Diagnosis is clinical. Delivery is recommended when gestational age is ≥ 34 weeks and is generally indicated... read more ).
Corticosteroids should also be considered in the following circumstances
At gestational age 34 weeks 0 days to 36 weeks 6 days if women are at risk of delivering within 7 days and no prior corticosteroids have been given (2 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more , 3 Treatment reference Prelabor rupture of membranes is leakage of amniotic fluid before onset of labor. Diagnosis is clinical. Delivery is recommended when gestational age is ≥ 34 weeks and is generally indicated... read more )
Starting at gestational age 23 weeks 0 days if there is a risk of preterm delivery within 7 days (2 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more , 3 Treatment reference Prelabor rupture of membranes is leakage of amniotic fluid before onset of labor. Diagnosis is clinical. Delivery is recommended when gestational age is ≥ 34 weeks and is generally indicated... read more ).
One of the following corticosteroids may be used:
Betamethasone 12 mg IM every 24 hours for 2 doses
Dexamethasone 6 mg IM every 12 hours for 4 doses
These corticosteroids accelerate maturation of fetal lungs and decrease risk of neonatal respiratory distress syndrome, intracranial bleeding, and mortality.
Progestins
A progestin has been recommended in future pregnancies for women who have a preterm delivery to reduce the risk of recurrence. This treatment is initiated during the 2nd trimester and continued until just before delivery.
However, supporting evidence is not definitive. Earlier studies showed meaningful reductions in preterm birth and neonatal morbidity for women who had had a preterm birth and were given 17-alpha-hydroxyprogesterone caproate (17-OHPC; 4 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ). But in a recent international study of women who had had a preterm birth, 17-OHPC was no more effective than placebo (5 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ). These discrepant results have triggered some controversy. The Society for Maternal-Fetal Medicine suggested that these discrepancies may reflect differences in the women sampled (eg, predominantly Caucasian and low-risk [6 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ]). The Society has called for additional studies but states that it is reasonable to treat women at very high risk of spontaneous preterm birth with 17-OHPC. Currently, the American College of Obstetricians and Gynecologists (ACOG) has reaffirmed its previous recommendations for the use of a progestin in future pregnancies when women are at risk of spontaneous recurrent preterm birth (7 Treatment references Labor (contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection... read more ). Clinicians should discuss risks and benefits of treatment with women at risk; then, decisions about treatment are made together.
Treatment references
1. American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics: Practice Bulletin No. 171: Management of Preterm Labor. Obstet Gynecol 128(4):e155-64. doi: 10.1097/AOG.0000000000001711
2. American College of Obstetricians and Gynecologists (ACOG): Prevention of group B streptococcal early-onset disease in newborns: ACOG Committee Opinion, Number 797. Obstet Gynecol 135 (2):e51–e72, 2020.
3. American College of Obstetricians and Gynecologists: Committee Opinion No. 713 Summary: Antenatal corticosteroid therapy for fetal maturation. Obstet Gynecol 130(2):493–494, 2017. doi: 10.1097/AOG.0000000000002231
4. Meis PJ, Klebanoff M, Thom E, et al: Prevention of recurrent preterm delivery by 17 alpha-hydroxyprogesterone caproate. N Engl J Med 348(24):2379–2385, 2003.
5. Blackwell SC, Gyamfi-Bannerman C, Biggio JR Jr, et al: 17-OHPC to prevent recurrent preterm birth in singleton gestations (PROLONG Study): A multicenter, international, randomized double-blind trial. Am J Perinatol Oct 25, 2019. doi: 10.1055/s-0039-3400227 [Epub ahead of print]
6. Society for Maternal-Fetal Medicine (SMFM) Publications Committee: SMFM Statement: Use of 17-alpha hydroxyprogesterone caproate for prevention of recurrent preterm birth. Accessed 4/23/21.
7. American College of Obstetricians and Gynecologists (ACOG): ACOG statement on FDA proposal to withdraw 17p hydroxyprogesterone caproate. 2020. Accessed 7/3/21.
Key Points
Do anovaginal cultures for group B streptococci and cultures to check for any clinically suspected infections that could have triggered preterm labor (eg, pyelonephritis, STDs).
Treat with antibiotics effective against group B streptococci pending culture results.
If the cervix dilates, consider tocolysis with magnesium sulfate, a calcium channel blocker, or, if the fetus is ≤ 32 weeks, a prostaglandin inhibitor.
Give a corticosteroid if the fetus is ≥ 24 weeks and < 34 weeks (in some cases < 37 weeks).
Consider giving corticosteroids starting at gestational age 23 weeks if there is a risk of preterm delivery within 7 days.
Consider magnesium sulfate if the fetus is < 32 weeks.
In future pregnancies, consider giving a progestin to prevent recurrence.
