Preterm Labor: Signs, Tocolytics and What Happens Next

Preterm birth -- delivery before 37 completed weeks of gestation -- affects approximately 9.9% of births worldwide; in the United States the rate holds at roughly 10%, according to the 2024 Global Burden of Disease analysis in The Lancet eClinicalMedicine. Preterm birth complications are the leading cause of under-five child mortality globally, accounting for approximately 1 million neonatal deaths in 2022 -- a figure that has shown little improvement over the past decade, underscoring why early recognition and rapid response matter so much.

If you are reading this because you felt something unusual today -- pressure you have not felt before, a cramping pattern that repeats, or fluid that was not there an hour ago -- please call your provider or go to labor and delivery before you finish reading. This article is for education and general guidance, not a substitute for clinical evaluation. This is general health information, not medical advice -- always consult your OB-GYN, midwife, or maternal-fetal medicine specialist about your individual situation.

What Are the Signs of Preterm Labor, and How Do I Know If What I'm Feeling Is Real?

Preterm labor is defined as regular uterine contractions accompanied by progressive cervical dilation and/or effacement before 37 weeks of gestation. The tricky part is that contractions alone -- even ones you can feel -- do not automatically mean your cervix is changing. But you should always report them to your provider rather than wait and see.

The warning signs ACOG and clinicians watch for include:

• Contractions occurring more than four times per hour -- even if they are mild, irregular-feeling, or do not match what you imagine labor should feel like
• Rhythmic low back pain or pressure -- a dull ache that comes and goes rather than staying constant, particularly if it radiates to the front
• Pelvic pressure -- a sensation that the baby is pushing down or that you feel very heavy in the pelvis
• Menstrual-like cramping in the lower abdomen, with or without diarrhea or nausea
• A change in vaginal discharge -- an increase in amount, or discharge that becomes watery, mucus-heavy, or tinged with pink or brown blood (bloody show)
• A gush or slow trickle of fluid -- this may indicate premature rupture of membranes (PROM), which precedes 25-40% of preterm deliveries according to published case-control data

One important caveat: Braxton Hicks contractions -- the irregular, practice contractions most pregnant women experience in the third trimester -- are common and benign. They tend to be irregular in timing, often ease when you change position or drink water, and do not strengthen or grow closer together over time. True preterm labor contractions follow a pattern: they come at measurable intervals, they do not resolve with rest or hydration, and they tend to become longer, stronger, and more frequent over time. When in doubt, time them for an hour. More than four in sixty minutes is your signal to call.

What Are Tocolytics and Antenatal Steroids, and How Do They Work Together?

When preterm labor is diagnosed, the two most important clinical interventions -- used together whenever possible -- are tocolytic therapy to suppress contractions and antenatal corticosteroids to accelerate fetal lung maturity. Understanding what each one does helps you understand why providers act quickly.

Tocolytics: Buying Time, Not Ending the Problem

Tocolytics are medications that relax the uterus and temporarily suppress contractions. Their goal is not to permanently stop preterm labor -- it is to create a 48-hour window in which antenatal steroids can work and, if necessary, the mother can be transferred safely to a facility with a neonatal intensive care unit capable of caring for a very preterm infant.

ACOG guidance is explicit: tocolytics are intended for short-term use only, not as a strategy for prolonged suppression of preterm labor. A 2025 comprehensive narrative review in Cureus confirmed that the four agents currently in clinical use are:

Tocolytic Agents Used in Preterm Labor Management

Agent	Class	Key Limitation	Typical Use Window
Nifedipine	Calcium-channel blocker	Maternal hypotension possible; avoid with certain cardiac conditions	24-34 weeks
Indomethacin	NSAID (COX inhibitor)	Contraindicated after 32 weeks -- can cause fetal renal impairment and premature closure of the ductus arteriosus	24-32 weeks
Terbutaline	Beta-2 agonist	Maternal tachycardia; FDA warns against use beyond 48-72 hours	Short-term only, any preterm gestation
Magnesium sulfate	Magnesium salt	Requires IV administration and close monitoring; given primarily for neuroprotection at <32 weeks	Under 32 weeks; also as neuroprotectant

No single tocolytic has been proven superior to the others across all outcomes. Your provider's choice will reflect your gestational age, your medical history, and the clinical setting.

Antenatal Corticosteroids: The Most Important 48 Hours

While the tocolytic works, your provider will administer antenatal corticosteroids -- either betamethasone (two 12 mg injections given 24 hours apart) or dexamethasone. These medications cross the placenta and trigger a cascade of fetal lung development that would otherwise take several more weeks. The result: dramatically reduced rates of respiratory distress syndrome (the leading cause of illness in premature newborns), intraventricular hemorrhage, and necrotizing enterocolitis.

Maximum benefit is achieved 24 to 48 hours after the first dose -- which is exactly why the 48-hour tocolytic window matters so much. Even a partial steroid course (at least one dose) confers meaningful benefit over no exposure at all, so providers will begin the series as soon as preterm labor is suspected, even before it's fully confirmed.

Can Preterm Birth Be Prevented? What the Evidence Says About Progesterone and Cervical Length

For women who have already had a preterm birth or who have a short cervix discovered on ultrasound, prevention is very much an active conversation. The most robustly supported intervention is vaginal progesterone -- and understanding why it works (and how it differs from its synthetic cousin) is worth the explanation.

Vaginal Progesterone for Short Cervix

If a transvaginal ultrasound between 18 and 24 weeks of pregnancy reveals a cervical length of 25 mm or less, ACOG recommends vaginal progesterone supplementation. A systematic review and meta-analysis across 14 high-quality randomized trials, cited in a 2023 evidence-based review in PubMed Central, found that vaginal progesterone in singleton pregnancies with a short cervix reduced preterm birth from 30.2% to 26.8% (relative risk 0.82; 95% CI 0.71-0.95), with associated reductions in neonatal mortality and respiratory distress syndrome. This is a clinically meaningful effect backed by high-certainty evidence.

Vaginal progesterone is a bioidentical formulation -- structurally identical to the progesterone your body produces. It is administered as a gel or pessary directly into the vagina, where it acts locally at the cervico-uterine interface through natural progesterone receptor pathways. This local action is believed to reduce inflammation, support cervical integrity, and maintain uterine quiescence through mechanisms that differ meaningfully from synthetic progestins.

The 17-OHPC Injection: What Changed in 2023

For many years, 17-alpha-hydroxyprogesterone caproate (17-OHPC or 17P) -- a synthetic progestin given as a weekly intramuscular injection -- was recommended for women with a prior spontaneous preterm birth. In April 2023, ACOG updated its guidance to no longer recommend intramuscular 17-OHPC for primary prevention of recurrent preterm birth in singleton pregnancies, following a large confirmatory trial that failed to replicate earlier positive findings. SMFM holds a conditional recommendation that diverges from this position. If you have a history of preterm birth, ask your provider specifically which approach they recommend and what the current trial evidence supports for your situation.

Cervical Cerclage for Cervical Insufficiency

For women with documented cervical insufficiency -- typically defined as one or more second-trimester losses attributed to painless cervical dilation without contractions, or a history of three or more second-trimester losses -- cervical cerclage is recommended. A stitch placed around the cervix provides mechanical support that vaginal progesterone alone cannot replicate. The 2025 AJOG MFM comparative systematic review on cerclage placement provides current evidence on timing and technique. Cerclage is typically placed at 12-14 weeks for history-indicated cases, or between 16 and 24 weeks when cervical shortening is found on surveillance ultrasound.

What Happens After a Preterm Birth Cannot Be Stopped?

When preterm birth is inevitable -- whether because labor is too advanced, membranes have ruptured, or a maternal or fetal complication requires early delivery -- the clinical team pivots to ensuring the best possible outcome for a premature newborn. Two interventions are standard when delivery appears imminent at very early gestations:

• Antenatal steroids (if not already given, or if the course is incomplete) -- even a partial course reduces the severity of respiratory complications
• Magnesium sulfate for neuroprotection -- given intravenously when preterm birth appears imminent before 32 weeks, magnesium sulfate significantly reduces the risk of cerebral palsy and major neurological injury in premature infants, per current Merck Manual clinical guidance

In a Level III or IV NICU, infants born as early as 23-24 weeks can survive with intensive support, though the road is long and the risks are real. Survival rates and long-term outcomes improve substantially with each additional week of gestation. At 28 weeks, survival exceeds 90% at major centers; at 32 weeks, the vast majority of infants do well with modest NICU support and are typically home within a few weeks of their original due date.

If you are facing a threatened preterm birth at a very early gestation, ask your care team to connect you with a neonatologist. A brief consultation before delivery -- even when time is limited -- can help you understand what to expect and make informed decisions alongside your providers.