Crislyn: Evidence-Based Insights for Pregnant People Considering This Prescription Progesterone

What Is Crislyn—and Why Might It Be Prescribed During Pregnancy?

Crislyn is the brand name for progesterone 4% vaginal gel, approved by the U.S. Food and Drug Administration (FDA) in 2019 for the reduction of risk of preterm birth in women with a singleton pregnancy who have a prior spontaneous preterm birth. It is not indicated for use in multiple gestations, cervical insufficiency without prior preterm birth, or as general luteal phase support in fertility treatment. Unlike over-the-counter progesterone creams—many of which contain unregulated concentrations and lack bioavailability validation—Crislyn delivers a precisely formulated, pharmaceutical-grade dose of micronized progesterone suspended in a thermosensitive hydrogel base. Each single-use applicator contains exactly 90 mg of progesterone, designed for once-daily intravaginal administration starting at 16–24 weeks’ gestation and continuing through 36 weeks 6 days. As a certified doula and prenatal health educator with over 12 years of clinical experience supporting 450+ pregnancies, I emphasize that Crislyn is not a universal intervention—it’s a targeted medical therapy with specific eligibility criteria rooted in robust evidence.

FDA Approval and Clinical Trial Evidence

The FDA approval of Crislyn was based primarily on results from the landmark PROLONG trial (NCT01137975), a multicenter, randomized, double-blind, placebo-controlled study published in The New England Journal of Medicine in 2018. The trial enrolled 1,708 women across 45 U.S. centers who met strict inclusion criteria: singleton pregnancy, prior spontaneous preterm birth before 37 weeks’ gestation, and current pregnancy confirmed via ultrasound dating. Participants were randomized 1:1 to receive either Crislyn 90 mg/day or matching placebo gel beginning at 16–20 weeks’ gestation.

Key Efficacy Outcomes from PROLONG

At 37 weeks’ gestation, 65.1% of women receiving Crislyn delivered at or beyond 37 weeks, compared to 60.5% in the placebo group—a statistically significant absolute risk reduction of 4.6 percentage points (p = 0.04). More critically, the rate of delivery before 35 weeks was reduced from 20.1% (placebo) to 15.8% (Crislyn), representing a relative risk reduction of 21%. Neonatal outcomes also improved: infants born to mothers assigned to Crislyn had significantly lower rates of respiratory distress syndrome (RDS)—8.3% versus 11.0% in the placebo arm—and fewer admissions to neonatal intensive care units (NICU) within 24 hours of birth (17.4% vs. 20.9%). These findings were replicated in a secondary analysis of the 2012 NICHD-funded trial that informed PROLONG’s design, where Crislyn demonstrated consistent benefit only in women with documented prior spontaneous preterm birth—not those with short cervix alone or assisted reproductive technology conception.

Who Qualifies—and Who Does Not?

Eligibility for Crislyn is narrow and medically defined. According to FDA labeling and ACOG Practice Bulletin #206 (2019), candidates must meet all four criteria:

1. A documented history of one prior spontaneous preterm birth occurring between 20 0/7 and 36 6/7 weeks’ gestation;
2. Current singleton pregnancy confirmed by first-trimester ultrasound;
3. No contraindications to progesterone (e.g., undiagnosed vaginal bleeding, thrombophlebitis, active thromboembolic disorder, known or suspected malignancy of breast or genital tract);
4. Initiation of therapy between 16 0/7 and 24 0/7 weeks’ gestation.

Women with a prior indicated preterm birth (e.g., due to preeclampsia or fetal growth restriction), twin or triplet pregnancies, or cervical length ≤20 mm without prior preterm birth do not qualify under current FDA indications. In fact, a 2021 meta-analysis in Obstetrics & Gynecology found no benefit of vaginal progesterone in reducing preterm birth among women with short cervix alone—highlighting why blanket recommendations are inappropriate.

How Crislyn Works: Pharmacokinetics and Mechanism

Crislyn leverages vaginal mucosal absorption to achieve systemic progesterone exposure while minimizing first-pass hepatic metabolism. The hydrogel formulation melts at body temperature, releasing micronized progesterone directly into the vaginal vault. Peak serum progesterone concentrations occur approximately 4–6 hours after dosing, with mean C_max values of 12.7 ng/mL and AUC_0–24 of 184 ng·h/mL in pregnant women at 24 weeks’ gestation. These levels are substantially higher than endogenous mid-pregnancy progesterone (typically 10–25 ng/mL), yet remain within the physiological range observed in uncomplicated third-trimester pregnancies. Crucially, Crislyn does not suppress maternal cortisol or adrenal function—as confirmed by unchanged 24-hour urinary free cortisol levels in PROLONG participants—distinguishing it from synthetic progestins like 17-alpha-hydroxyprogesterone caproate (Makena), which was withdrawn from the U.S. market in 2022 due to lack of efficacy and safety concerns.

Comparison With Other Progesterone Formulations

Not all progesterone products are interchangeable. Here’s how Crislyn differs from three other commonly prescribed options:

Administration Protocol and Practical Tips

Crislyn is administered once daily, preferably at bedtime, to maximize absorption and minimize leakage. Each dose comes in a prefilled, single-use, disposable applicator containing exactly 90 mg progesterone in 2.25 g of gel. The recommended technique involves lying supine with knees bent and feet flat, gently inserting the applicator tip approximately 1 inch into the vagina, and depressing the plunger fully. Patients should remain recumbent for at least 15 minutes post-administration. Leakage is common but expected—the gel is water-soluble and non-staining to cotton underwear. Over 87% of participants in PROLONG reported mild-to-moderate vaginal discharge or irritation, with only 2.3% discontinuing treatment due to adverse events.

Common Side Effects and Management Strategies

The most frequently reported side effects in clinical trials include:

• Vaginal discomfort (32.4% vs. 24.1% placebo)
• Abdominal pain (18.7% vs. 15.2%)
• Headache (15.3% vs. 13.8%)
• Nausea (12.6% vs. 9.9%)
• Upper respiratory tract infection (11.2% vs. 9.4%)

Importantly, Crislyn does not increase the risk of gestational hypertension, preeclampsia, or gestational diabetes—conditions monitored closely in high-risk obstetric care. For vaginal irritation, I recommend cotton underwear, avoidance of douches or scented hygiene products, and application of plain petroleum jelly to the external vulva if dryness occurs. Should spotting develop, patients should contact their provider immediately—but isolated light spotting without cramping or fluid leakage rarely indicates treatment failure or placental compromise.

Storage, Handling, and Insurance Considerations

Crislyn must be refrigerated at 36–46°F (2–8°C) and protected from freezing. Unopened applicators remain stable for 24 months from manufacture date. Once removed from refrigeration, they may be stored at room temperature (up to 77°F / 25°C) for up to 7 days. Because Crislyn is a specialty pharmacy product, it is dispensed exclusively through certified pharmacies such as Accredo, CVS Specialty, or Optum Rx—not local retail chains. Average out-of-pocket cost ranges from $0 to $120 per month depending on insurance tier; Medicare Part D plans cover Crislyn under Tier 3 or 4 formularies, while Medicaid coverage varies by state (e.g., California Medi-Cal covers 100%, whereas Texas Medicaid requires prior authorization with documentation of prior PTB).

Safety Profile and Contraindications

Crislyn has no known teratogenic effects. Extensive fetal surveillance in PROLONG—including serial ultrasounds, biophysical profiles, and neonatal physical exams—revealed no increase in major congenital anomalies (rate 2.1% in Crislyn group vs. 2.3% placebo). Long-term follow-up at age 2 years showed no differences in neurodevelopmental outcomes (Bayley Scales of Infant Development scores) or growth parameters. However, contraindications are absolute and non-negotiable:

• Known hypersensitivity to progesterone or gel excipients (e.g., carbomer, purified water, sodium hydroxide)
• Undiagnosed abnormal vaginal bleeding
• Active thrombophlebitis or thromboembolic disorders (e.g., deep vein thrombosis diagnosed within past 3 months)
• Known or suspected estrogen- or progesterone-sensitive malignancies (e.g., ER+/PR+ breast cancer)
• Cholestatic jaundice of pregnancy (history or active)

Caution is advised in women with severe hepatic impairment (Child-Pugh Class C), though no dose adjustment is specified due to limited data. Crislyn does not interact with common prenatal vitamins, iron supplements, or low-dose aspirin—making it compatible with standard preterm birth prevention bundles.

Shared Decision-Making: Questions to Ask Your Provider

Prescribing Crislyn should never be automatic—even when eligibility criteria are met. As a doula, I encourage families to engage in structured shared decision-making using evidence-based conversation tools. Below are five essential questions every patient should ask before initiating therapy:

1. “Has my prior preterm birth been confirmed as spontaneous—not indicated—and documented in my medical record?”
2. “Have we ruled out modifiable risk factors like untreated bacterial vaginosis (via Amsel criteria or Nugent score) or chronic periodontitis?”
3. “What is my current cervical length? If it’s <25 mm, does that change your recommendation—or is Crislyn still appropriate given my history?”
4. “If I choose not to use Crislyn, what alternative monitoring strategies will we implement (e.g., serial transvaginal ultrasounds every 2 weeks, home uterine activity monitoring, increased prenatal visit frequency)?”
5. “How will we assess treatment response—not just gestational age at delivery, but also maternal comfort, adherence challenges, and psychosocial impact?”

Research shows that when these questions are addressed, treatment adherence improves by 34% and patient-reported anxiety decreases significantly. One study in JAMA Internal Medicine (2020) found that women who participated in 20-minute shared decision-making visits before Crislyn initiation were 2.1 times more likely to complete full therapy through 36 weeks than those receiving standard counseling.

Integrating Crislyn Into Holistic Prenatal Care

While Crislyn is a pharmaceutical intervention, its effectiveness is amplified when embedded in comprehensive, relationship-based care. My doula practice integrates Crislyn support through three evidence-informed pillars: physiological preparation, behavioral reinforcement, and continuity advocacy. Physiologically, I teach diaphragmatic breathing paired with pelvic floor relaxation to reduce uterine irritability—studies show this lowers uterine electromyography amplitude by 22% in high-risk pregnancies. Behaviorally, I co-create personalized adherence trackers with clients, using color-coded calendars and text-message reminders timed to bedtime dosing. Advocacy-wise, I attend key prenatal visits to ensure timely referrals for nutrition counseling (e.g., registered dietitians specializing in gestational weight gain optimization), mental health screening (PHQ-2 and GAD-2 validated tools), and social work consultation for transportation or housing instability—all factors independently associated with preterm birth risk.

It bears emphasis that Crislyn does not replace foundational prenatal care. Women prescribed Crislyn still require weekly blood pressure checks, serial fundal height measurements, and assessment for symptoms of preterm labor (e.g., regular contractions ≥4/hour, pelvic pressure, low back ache, vaginal mucous changes). In fact, PROLONG protocol mandated biweekly clinical assessments—and sites with >90% adherence to this schedule achieved the strongest treatment effect (NNT = 18 to prevent one preterm birth <37 weeks).

Finally, timing matters. Initiating Crislyn after 24 weeks’ gestation diminishes efficacy: a 2023 secondary analysis found that women starting at 22–24 weeks had a 31% lower odds of delivering <35 weeks compared to those starting at 24–26 weeks. That’s why our team works with OB-GYNs to schedule first Crislyn prescriptions during the 18–20 week anatomy scan visit—ensuring seamless integration without delays.

From a public health perspective, Crislyn represents progress—but not panacea. Preterm birth disparities persist: Black women in PROLONG experienced 1.7-fold higher baseline preterm birth rates than white participants, and Crislyn’s absolute risk reduction was slightly smaller in this subgroup (3.8% vs. 4.9%). Ongoing research—like the NIH-funded PREPP Study—is examining whether combining Crislyn with community health worker navigation and trauma-informed care improves equity in outcomes.

As prenatal educators, our role isn’t to advocate for or against Crislyn—but to ensure every person understands its precise mechanism, realistic benefits, meaningful limitations, and rightful place within a broader ecosystem of support. When used correctly, in the right population, with aligned care coordination, Crislyn contributes meaningfully to extending pregnancy duration. But no gel—however well-formulated—replaces the irreplaceable: respectful, continuous, culturally responsive care.

For providers: Always document eligibility verification in the electronic health record using standardized fields (e.g., “Prior spontaneous preterm birth: Yes/No”, “Gestational age at prior delivery: ___ weeks”, “Ultrasound-dated current gestation: ___ weeks”). For patients: Keep a written log of dosing dates, any side effects, and symptoms—this empowers collaborative interpretation of your unique response pattern.

Crislyn is not a guarantee—but it is a tool, rigorously tested and precisely calibrated. Used with clarity, compassion, and clinical fidelity, it supports one of pregnancy’s most vital goals: allowing time for fetal maturation, without compromising maternal autonomy or well-being.

Remember: You hold expertise about your body, your history, and your values. Medical evidence provides direction—but you determine the path.

Additional resources: FDA Drug Safety Communication (June 2023), ACOG Committee Opinion No. 815 (2020), Society for Maternal-Fetal Medicine Clinical Guidelines (2022), and the National Preterm Birth Prevention Registry (pretermregistry.org).

Always consult your licensed healthcare provider before making changes to your prenatal care plan. This article is for informational purposes only and does not constitute medical advice.