Preterm Labor & Premature Rupture of Membranes

Preterm labor: definition, risk, and prediction

Definition. Preterm labor is regular uterine contractions accompanied by cervical change (dilation, effacement, or both) before 37 0/7 weeks, or presentation with regular contractions and cervical dilation of at least 2 cm. About 10% of US births are preterm. Contractions without cervical change are not labor, and roughly half of women admitted for preterm labor still deliver at term, which is why overtreatment is a real risk.

Risk factors. A prior spontaneous preterm birth is the strongest predictor, especially when it occurred early. Others:

• Short cervix on transvaginal ultrasound; prior cervical excision (LEEP, cone biopsy).
• Multiple gestation, polyhydramnios, or uterine anomalies (overdistension or distortion).
• Infection: intraamniotic infection, untreated asymptomatic bacteriuria or pyelonephritis.
• Vaginal bleeding, including abruption.
• Smoking, short interpregnancy interval, and low prepregnancy weight.

Prediction. In a symptomatic woman at 24–34 weeks, two tests help mainly by their negative predictive value. A negative cervicovaginal fetal fibronectin, or a transvaginal cervical length of 30 mm or more, makes delivery within the next 7 days very unlikely, sparing steroids, tocolysis, and transfer. A positive fibronectin is a poor predictor. The fibronectin swab is invalid after intercourse within 24 hours, vaginal bleeding, lubricant or a digital exam, ruptured membranes, or dilation of 3 cm or more, so collect it before examining the cervix.

Bed rest, hydration, and pelvic rest have not been shown to prevent preterm birth, and bed rest adds thromboembolic risk.

Acute management of preterm labor

Antenatal corticosteroids. If delivery within 7 days is likely at 24 0/7–33 6/7 weeks, give betamethasone 12 mg IM in two doses 24 hours apart, or dexamethasone 6 mg IM every 12 hours for four doses. They may be considered from 22–23 weeks when neonatal resuscitation is planned. Steroids accelerate surfactant production by type II pneumocytes and reduce neonatal death, respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis; the benefit peaks from 48 hours to 7 days after the first dose. Before 34 weeks, a single rescue course may be given if the first was at least 14 days earlier and delivery is again likely within 7 days.

Late preterm steroids. At 34 0/7–36 6/7 weeks, a woman likely to deliver within 7 days who has never received steroids may be given betamethasone, which reduces neonatal respiratory morbidity. Tocolysis is never used to complete a late-preterm course. These steroids raise the rate of neonatal hypoglycemia, and women with pregestational diabetes were excluded from the ALPS trial that supports them, so SMFM advises against late preterm steroids in pregestational diabetes.

Magnesium for neuroprotection. When delivery before 32 0/7 weeks is anticipated, IV magnesium sulfate reduces the risk and severity of cerebral palsy in surviving infants. It is not an effective tocolytic. Because it is renally cleared, monitor patellar reflexes, respiratory rate, and urine output. Toxicity progresses from loss of reflexes to respiratory depression to cardiac arrest; stop the infusion and give calcium gluconate. Magnesium is avoided in myasthenia gravis.

Tocolysis. Tocolytics can delay delivery for up to 48 hours at roughly 24–34 weeks, enough to complete steroids, give magnesium, or transfer to a center with a NICU. They have not been shown to improve neonatal outcomes by themselves, and maintenance tocolysis after an episode is not recommended. Contraindications: intrauterine fetal demise or lethal anomaly, nonreassuring fetal status, preeclampsia with severe features or eclampsia, maternal hemorrhage with instability, intraamniotic infection, and maternal contraindications to the specific drug.

GBS prophylaxis. In preterm labor, collect a rectovaginal GBS culture and start IV penicillin G (ampicillin as the alternative) unless a culture from the past 5 weeks was negative. Stop when labor stops or the culture returns negative. Other antibiotics do not prolong pregnancy when membranes are intact and are not given for that purpose.

Prevention: progesterone and cerclage

Vaginal progesterone. For a singleton pregnancy without a prior preterm birth, a transvaginal cervical length of 20 mm or less at or before 24 weeks is treated with daily vaginal progesterone until about 36 weeks, which reduces preterm birth and neonatal morbidity. Cervical length is usually assessed at the 18–22-week anatomy scan. Women with a prior spontaneous preterm birth have serial cervical length measurements from 16 to 24 weeks.

Weekly IM 17-hydroxyprogesterone caproate, once standard for prior preterm birth, was withdrawn from the US market in 2023 after a large trial showed no benefit.

Limits of cerclage. It is contraindicated with active labor, ruptured membranes, intraamniotic infection, active bleeding, or a lethal fetal anomaly, and it is not recommended for a short cervix in twin pregnancies.

Rupture of membranes: diagnosis

Definitions. Prelabor rupture of membranes (PROM) is rupture before labor begins; preterm PROM (PPROM) is rupture before 37 weeks and accounts for about one-third of preterm births. Risk factors overlap with preterm labor: prior PPROM, genital tract infection, bleeding, and smoking.

Sterile speculum exam. A history of a gush or steady leak is suggestive but not diagnostic. Look for pooling of fluid in the posterior fornix (ask her to cough or bear down), a nitrazine test that turns blue because amniotic fluid is alkaline (pH about 7.0–7.5, versus about 4.5 in the vagina), and ferning when the fluid dries on a slide. Nitrazine can be falsely positive with blood, semen, bacterial vaginosis, trichomoniasis, or antiseptics, and cervical mucus can fern. Oligohydramnios on ultrasound supports the diagnosis; if findings stay equivocal, an immunoassay for placental alpha-microglobulin-1 helps.

No digital exams. Unless she is in active labor or delivery is planned, avoid digital cervical exams: they carry bacteria upward, shorten latency, and raise infection risk. Estimate dilation visually during the speculum exam and confirm fetal presentation by ultrasound, since malpresentation raises the risk of cord prolapse.

Rupture of membranes: management and complications

Latency antibiotics. For PPROM before 34 weeks, a 7-day course of IV ampicillin plus erythromycin for 48 hours, followed by oral amoxicillin plus erythromycin for 5 days, prolongs the time to delivery and lowers chorioamnionitis and neonatal infection. A single dose of azithromycin may replace erythromycin. Amoxicillin-clavulanate is avoided because it increased necrotizing enterocolitis in the ORACLE trial.

When to end expectant management. Deliver at 34 0/7 weeks, or sooner for intraamniotic infection, placental abruption, nonreassuring fetal status, cord prolapse, or labor. About half of patients deliver within a week of rupture regardless. Monitoring includes maternal temperature and pulse, uterine tenderness, and fetal heart rate testing.

Intraamniotic infection (chorioamnionitis). Suspect it with maternal fever plus fetal tachycardia (>160/min), maternal leukocytosis, uterine tenderness, or purulent, foul-smelling fluid. Treat with IV ampicillin plus gentamicin and deliver, usually by induction; cesarean is reserved for standard obstetric indications, and clindamycin or metronidazole is added after a cesarean for anaerobic coverage. Tocolysis is contraindicated.

Very early rupture. Rupture before about 24 weeks with persistent oligohydramnios adds pulmonary hypoplasia and limb contractures to the hazards of extreme prematurity. At any gestational age, PPROM also carries risks of abruption and cord compression or prolapse.