Labor (regular uterine contractions resulting in cervical change) that begins before 37 weeks gestation is considered preterm. Risk factors include prelabor rupture of membranes, uterine abnormalities, infection, prior preterm birth, multiple gestation, and fetal or placental abnormalities. Diagnosis is clinical. Causes are identified and treated if possible. Management typically includes bed rest, tocolytics (if labor persists), corticosteroids (eg, if gestational age is 34 weeks—see below) and possibly magnesium sulfate (if gestational age is 32 weeks). Antistreptococcal antibiotics are given pending negative anovaginal culture results for group B streptococci.
Preterm labor may be triggered by
Intra-amniotic infection (chorioamnionitis)
Another ascending uterine infection (commonly due to group B streptococci)
Fetal or placental abnormalities
Uterine abnormalities
Pyelonephritis
Some sexually transmitted infections (STIs)
A cause may not be evident.
Prior preterm delivery and cervical incompetence increase the risk of preterm delivery.
Preterm labor can increase risk of intraventricular hemorrhage in neonates; intraventricular hemorrhage may result in neurodevelopmental disability (eg, cerebral palsy).
Diagnosis of Preterm Labor
History and physical examination
Diagnosis of preterm labor is based on signs of labor and length of the pregnancy.
Anovaginal cultures for group B streptococci are done, and antibiotic prophylaxis is initiated (and then discontinued if cultures are negative). Urinalysis and urine culture are done to check for cystitis and pyelonephritis. Cervical cultures are done to check for STIs if suggested by risk factors and if the patient has not been tested recently.
Many women with preterm contractions are not in labor, and some women diagnosed with preterm labor do not progress to delivery.
Treatment of Preterm Labor
Antibiotics for group B streptococci
Sometimes tocolytics
Corticosteroids (eg, between 24 and 34 weeks)
Magnesium sulfate for neuroprotection
Management of preterm labor includes antibiotics (if infection is diagnosed or suspected), tocolytics, and corticosteroids (1).
Antibiotics
Antibiotics effective against group B streptococci are given pending negative anovaginal cultures (2). Choices for antibiotics include the following:
Urinary tract infections and STIs are treated, if diagnosed.
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
A calcium channel blocker
Prostaglandin inhibitors
No tocolytic is clearly the first-line choice; choice should be individualized to minimize adverse effects.
Prostaglandin inhibitors may cause transient oligohydramnios and fetal renal damage if used for more than 48 consecutive hours. They are contraindicated after 32 weeks gestation because they may cause premature narrowing or closure of the ductus arteriosus.
Magnesium sulfate
IV magnesium sulfate should be considered in pregnancies < 32 weeks for the purpose of neuroprotection. 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.
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.
Corticosteroids should also be considered in the following circumstances
At 34 0/7 weeks to 36 6/7 weeks gestation if women are at risk of delivering within 7 days and no prior corticosteroids have been given (2, 3)
Starting at 23 0/7 weeks gestation if there is a risk of preterm delivery within 7 days (2, 3).
At 22 0/7 weeks to 22 6/7 weeks gestation if neonatal resuscitation is planned and after appropriate parental counseling (4)
One of the following corticosteroids may be used:
These corticosteroids accelerate maturation of fetal lungs and decrease risk of neonatal respiratory distress syndrome, intracranial bleeding, and mortality.
Progestins
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): ACOG Committee Opinion, Number 797: Prevention of group B streptococcal early-onset disease in newborns. Obstet Gynecol 135 (2):e51–e72, 2020. Reaffirmed 2022.
3. American College of Obstetricians and Gynecologists (ACOG): ACOG Committee Opinion No. 713 Summary: Antenatal corticosteroid therapy for fetal maturation. Obstet Gynecol 130(2):493–494, 2017. doi: 10.1097/AOG.0000000000002231. Reaffirmed 2024.
4. American College of Obstetricians and Gynecologists (ACOG): ACOG Practice Advisory: Use of Antenatal Corticosteroids at 22 Weeks of Gestation, September 2021. Reaffirmed October 2022.
5. Conde-Agudelo A, Romero RAm J Obstet Gynecol 227(3):440-461.e2, 2022. doi:10.1016/j.ajog.2022.04.023
6. Society for Maternal-Fetal Medicine (SMFM)Am J Obstet Gynecol 229(1):B2-B6, 2023. doi:10.1016/j.ajog.2023.04.012
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, STIs).
Treat with antibiotics effective against group B streptococci pending culture results.
If the cervix dilates, consider tocolysis with 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.
