Preterm labor: Clinical sciences

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Introduction0:00–0:53

Preterm labor is defined as regular uterine contractions accompanied by a change in cervical dilation, effacement, or both, that occurs between 20 and 0/7 weeks of gestation and 36 and 6/7 weeks of gestation.
It’s diagnosed when a patient presents with regular uterine contractions and a cervical dilation of at least 3 cm. Preterm labor can be either spontaneous or preceded by premature rupture of membranes; or it can be iatrogenic due to maternal or fetal causes necessitating early delivery.
The majority of patients who initially present with evidence of preterm labor do not deliver prematurely; however, when labor progresses to preterm birth, there’s an increased risk of neonatal morbidity and mortality.
When a patient presents with a chief concern suggesting preterm labor, your first step is to perform a focused history and physical exam.

History and physical0:53–2:51

To meet criteria for preterm labor your patient’s pregnancy must be at least 20 weeks and less than 37 weeks of gestation.
They may report uterine contractions, abdominal cramps, low back pain, pelvic pressure, spotting, or vaginal bleeding. Additionally, history might reveal some important risk factors for preterm birth, including a prior preterm birth; short interval pregnancy, defined as less than 18 months between deliveries; a known short cervix, meaning less than 25 mm in the current pregnancy; past procedures on the cervix; infections during the current pregnancy, such as urinary or genital tract infections; maternal prepregnancy body mass index less than 18.5; extremes of maternal age, specifically those younger than 17 or older than 35 years; and tobacco or substance use during pregnancy.
Also, certain social determinants of health are associated with preterm birth, specifically lower levels of education, living in an disadvantaged area, and lack of access to prenatal care.
An additional important consideration is that preterm birth is higher for Non-Hispanic Black and Indigenous individuals.Here’s a clinical pearl!
The risk factors discussed refer to singleton pregnancies; however, multifetal gestations are at an increased risk of preterm labor and birth as well.On physical exam, you’ll find regular uterine contractions, intact amniotic membranes, and possible vaginal bleeding.
If your patient has ruptured membranes they are diagnosed with preterm, premature rupture of membranes, or PPROM, and not preterm labor.
Okay, if your patient is at least 20 weeks but less than 34 weeks of gestation, their labor is considered early preterm.

Early preterm2:51–10:27

Your next step is to obtain a fetal fibronectin, or fFN, specimen, by performing a sterile speculum exam and collecting a sample of cervicovaginal secretions from the posterior vaginal fornix.
Fetal fibronectin is a protein that acts like a “glue” between the amniotic membranes and the uterus. Normally, fFN is found at low levels in cervicovaginal secretions; however, when found at higher levels, its presence is associated with an increased risk of spontaneous preterm birth.
That being said, the positive predictive value of fFN is quite low, meaning that when used alone it is not a reliable indicator of which patients will go into preterm labor.
Rather, the utility of the fFN test is in its high negative predictive value, so when the fFN is negative in a patient with preterm labor symptoms, you can be assured that your patient is not experiencing preterm labor.
Moreover, several factors can interfere with the accuracy of the fFN test. These include the use of lubricants, soaps, and vaginal progesterone; moderate or gross vaginal bleeding; and the presence of semen or sperm, most commonly found if the patient was sexually active within 24 hours.
If these are present, the specimen should not be collected. Next, assess cervical dilation.
If your patient is at least 3 centimeters dilated, diagnose preterm labor and admit them to labor and delivery. Now it's time to begin interventions that can help decrease neonatal morbidity and mortality.
First administer antenatal steroids, which promote fetal lung maturation, and decrease the risk of respiratory distress syndrome, intracranial hemorrhage, necrotizing enterocolitis, and death.
There are two steroids available which are administered intramuscularly: dexamethasone which is given in 4 doses every 12 hours, and betamethasone which is given in 2 doses 24 hours apart.
A full course has the greatest benefit and a patient is considered to be “steroid complete” 48 hours after the first dose of medication.
Even in instances where delivery is imminent and the patient will not receive the full course, there is still a benefit to giving a partial course of steroids.
Next, obtain a GBS culture and begin antibiotics for GBS prophylaxis, generally either IV penicillin or ampicillin. Continue the antibiotics until delivery or if cultures return negative.
Additionally, if your patient is between 24 and 32 weeks of gestation, administer IV magnesium sulfate for fetal neuroprotection, as it reduces the risk and severity of cerebral palsy.
Finally, begin tocolysis by administering medications to inhibit uterine contractions. The goal of tocolysis is to allow for a complete course of antenatal steroids, administration of magnesium sulfate, and transport to a tertiary facility if indicated.
Accordingly, tocolysis should be continued for up to 48 hours, when possible. Here’s another clinical pearl!
There are some situations where tocolysis is contraindicated. These include intrauterine fetal demise; lethal fetal anomaly; non-reassuring fetal status; intraamniotic infection; PPROM; maternal bleeding with hemodynamic instability such as with placental abruption; severe preeclampsia or eclampsia; and if a maternal contraindication to a specific medication used for tocolysis is present.
Tocolytic options include calcium channel blockers, or CCBs; non-steroidal anti-inflammatory drugs, or NSAIDs; beta-adrenergic receptor agonists; and magnesium sulfate.
CCBs, like nifedipine, may cause dizziness, flushing, hypotension, and decreased maternal heart rate, so CCBs are contraindicated in patients with hypotension or preload-dependent cardiac lesions, like aortic insufficiency.
NSAIDs, like indomethacin, are safe for patients less than 32 weeks of gestation but are associated with in-utero constriction of the fetal ductus arteriosus and oligohydramnios, as well as necrotizing enterocolitis and patent ductous arteriosus in newborns.
Contraindications to NSAIDs include platelet, renal, or hepatic dysfunction; a history of gastrointestinal ulcers; and asthma.
Next, beta-adrenergic receptor agonists, like terbutaline, have side effects including tachycardia, hypotension, tremor, palpitations, shortness of breath, electrolyte changes, chest discomfort, and pulmonary edema; as well as fetal tachycardia.
Beta-adrenergic receptor agonists should be avoided in patients with poorly controlled diabetes and tachycardia-sensitive cardiac disease.
Lastly, magnesium sulfate has been shown to decrease uterine activity and can be used as a tocolytic if also being used for fetal neuroprotection.
However, if contractions persist, use a different tocolytic instead. Side effects of magnesium sulfate include flushing, loss of deep tendon reflexes, and respiratory depression; and it’s contraindicated in patients with myasthenia gravis.
Finally, obtain a neonatology consultation, as premature newborns potentially have significant morbidity and mortality.Okay, let’s now go back and review what to do if your patient’s cervix is less than 3 cm dilated.
Your next step is to obtain a transvaginal ultrasound to assess cervical length. Also, consider sending your previously collected fFN for evaluation, specifically if the cervical length on ultrasound is between 20 and 30 mm OR if transvaginal ultrasound is not readily available.
In all other scenarios, you can discard the collected specimen.If the cervical length is less than 20 mm or it is 20 to 30 mm with a positive fFN, diagnose preterm labor.
Again, admit the patient to labor and delivery, administer antenatal steroids, start GBS prophylaxis, provide neuroprotection with magnesium sulfate if gestational age is between 24 and 32 weeks, initiate tocolysis, and obtain a neonatology consultation.
However, if the cervical length is 20 to 30 mm with a negative fFN OR the cervical length is greater than 30 mm you will observe your patient.
Repeat their sterile vaginal exam in 6 to 12 hours, or earlier if they appear to be progressing in labor. If there is no change in the cervical exam, diagnose threatened preterm labor and discharge your patient home with labor precautions.
Alternatively, if there is a change in the cervical exam, diagnose preterm labor, admit your patient to labor and delivery, administer antenatal steroids, start GBS prophylaxis, provide neuroprotection with magnesium sulfate if gestational age is between 24 and 32 weeks, initiate tocolysis, and obtain a neonatology consultation.
Now let’s discuss patients who are at least 34 weeks but less than 37 weeks of gestation. This timeframe is considered the late preterm period.

Late preterm10:27–11:46

Your first step is to assess cervical dilation and repeat the sterile vaginal exam in 4 to 6 hours. If there is no change, diagnose threatened preterm labor and discharge your patient with labor precautions.Alternatively, if you observe a change in cervical exam, diagnose preterm labor.
Admit your patient to labor and delivery, administer a course of antenatal steroids, and consider GBS prophylaxis. Patients presenting in week 36 of pregnancy may have already had their GBS culture completed; and if its negative, prophylaxis is not indicated.
Otherwise, begin IV antibiotics until delivery or a negative culture result. In these patients, proceed with expectant management of labor, meaning neither augmentation nor tocolysis is indicated.
Finally, obtain a neonatology consultation.One last clinical pearl! Patients who present with preterm labor between 20 and 23 weeks and 6 days of gestation require individualized counseling with neonatology and maternal-fetal medicine.
Alright, as a quick recap… For those in the early preterm period, a cervical exam, transvaginal ultrasound, and possibly fFN will differentiate between threatened and active preterm labor.

Review11:46–12:11

For those in the late preterm period, serial cervical exams will help make your diagnosis. Preterm patients are managed with antenatal steroids, GBS prophylaxis, magnesium sulfate, and tocolysis.