What is the best course of action for a 13-week pregnant individual with an itchy rash on their legs and arms?

Management of Itchy Rash at 13 Weeks Gestation

Immediately measure serum bile acids and liver transaminases to rule out intrahepatic cholestasis of pregnancy (ICP), even though a visible rash is present, because excoriations from scratching can mimic a primary rash and missing ICP delays critical interventions that prevent stillbirth. 1

Initial Diagnostic Approach

The first critical step is determining whether this represents a true primary rash or pruritus with secondary skin changes from scratching:

• If a true primary rash is present (urticarial papules, plaques, or eczematous changes), the most likely diagnosis at 13 weeks is atopic eruption of pregnancy (AEP), which is the most common pruritic disorder in pregnancy and presents with eczematous rash on the face, neck, trunk, and extremities 2

• If pruritus predominates without a clear primary rash, or if the rash appears to be secondary excoriations, ICP must be ruled out immediately despite the early gestational age, as it carries significant stillbirth risk 2, 3, 1

Mandatory Laboratory Testing

Order the following tests regardless of whether a rash is visible:

• Serum bile acids (total): Diagnostic threshold >10 μmol/L for ICP 3, 1
• Liver transaminases (ALT, AST): May be elevated in ICP but not required for diagnosis 3, 1
• The turnaround time ranges from 4 hours to 14 days depending on assay availability 1

Management Based on Diagnosis

If Atopic Eruption of Pregnancy (AEP) is Diagnosed:

• Apply moderate-potency topical corticosteroids to affected areas for symptom relief 1
• Use emollients liberally to prevent skin dryness 4, 1
• Advise loose, breathable clothing 1
• This condition is benign to both mother and fetus with no associated fetal risks 1
• Symptoms typically resolve within 6 weeks postpartum 1

If Intrahepatic Cholestasis of Pregnancy (ICP) is Confirmed:

• Initiate ursodeoxycholic acid (UDCA) 10-15 mg/kg/day in divided doses immediately as first-line treatment (GRADE 1A recommendation) 2, 3
• UDCA improves pruritus, serum bile acid levels, liver transaminases, and meta-analyses demonstrate decreased adverse outcomes including preterm birth and stillbirth 3
• Begin antenatal fetal surveillance at a gestational age when delivery would be performed in response to abnormal testing 2
• Plan delivery timing based on bile acid levels:
  • Bile acids ≥100 μmol/L: deliver at 36 0/7 weeks (GRADE 1B) 2, 3
  • Bile acids <100 μmol/L: deliver between 36 0/7 and 39 0/7 weeks (GRADE 1C) 2, 3
• If UDCA alone is insufficient for pruritus control, add cholestyramine as second-line therapy 3
• Monitor PT/INR regularly if prescribing cholestyramine, as it can cause vitamin K deficiency 1

Symptomatic Treatment While Awaiting Results:

• Chlorphenamine is the preferred antihistamine during pregnancy due to its long safety record, though it's best to avoid all antihistamines in the first trimester if possible 4
• Cetirizine (FDA Pregnancy Category B) may be used if necessary, using the lowest effective dose 4
• Non-pharmacological measures include:
  • Emollients to prevent skin dryness 4
  • Avoiding hot baths or showers 4
  • Using cooling gels 4
  • Keeping nails shortened to minimize excoriation 4

Critical Pitfalls to Avoid

• Never delay bile acid testing in any pregnant patient with pruritus, even if a rash is present, as excoriations from scratching can mimic a primary rash 1
• Do not dismiss early-onset pruritus as benign without laboratory confirmation—while ICP typically presents in the second or third trimester, it can occur earlier 2, 3
• Do not deliver preterm based on clinical suspicion of ICP alone without laboratory confirmation of elevated bile acids (GRADE 1B) 2, 1
• Avoid betamethasone or dexamethasone for pregnancy dermatoses, as they cross the placenta readily; use prednisolone instead if systemic corticosteroids are needed 1

Differential Diagnosis Considerations at 13 Weeks

At this early gestational age, the differential includes:

• Atopic eruption of pregnancy (AEP): Most common, presents with eczematous rash 2, 1
• Intrahepatic cholestasis of pregnancy (ICP): Less common at 13 weeks but must be excluded due to fetal risks 2, 3
• Polymorphic eruption of pregnancy (PEP): Rare at 13 weeks, typically occurs in third trimester and primigravidas 1, 5
• Pemphigoid gestationis (PG): Rare, presents with vesicles and bullae, requires immediate dermatology referral and immunofluorescence studies 1
• Non-pregnancy specific causes: Scabies, contact dermatitis, drug reactions, viral exanthems 6, 7

Follow-Up

• If bile acids are normal but clinical suspicion remains high, repeat testing after excluding other causes including biliary obstruction, viral hepatitis, and chronic liver disease 3
• If pruritus persists beyond 6 weeks postpartum, pursue further evaluation for underlying chronic liver disease 3