What could be causing lower abdominal pain below the belly button, radiating to the hip bone, that occurs a week after menstruation in a postpartum patient?

Postpartum Cyclic Lower Abdominal Pain: Differential Diagnosis and Evaluation

The most likely causes of cyclic lower abdominal pain occurring one week after menstruation in a postpartum patient include endometriosis, adenomyosis, ovarian cysts, or pelvic adhesions from the delivery, with transvaginal and transabdominal ultrasound being the appropriate first-line imaging to establish the diagnosis. 1, 2, 3

Understanding the Clinical Pattern

The timing of pain one week after menstruation (mid-cycle) suggests several possibilities:

• Ovarian cysts are the most common gynecologic cause of pelvic pain in reproductive-age women, accounting for approximately one-third of cases, and can cause mid-cycle pain related to ovulation or cyst rupture 1, 3
• Endometriosis is an estrogen-dependent condition causing chronic cyclic pain that correlates with hormonal fluctuations throughout the menstrual cycle 2
• Adenomyosis causes chronic pelvic pain and may worsen postpartum due to uterine changes from pregnancy 2
• Pelvic adhesions from cesarean delivery or vaginal delivery complications can cause chronic pain, though the cyclic nature makes this less likely as the sole cause 1, 2

Recommended Diagnostic Approach

Begin with transvaginal ultrasound combined with transabdominal ultrasound as the initial imaging study. 1 This approach provides:

• High-frequency detailed views of the uterus and ovaries (transvaginal) 1
• Broader field of view to assess the entire pelvis and detect free fluid (transabdominal) 1
• Color Doppler evaluation to assess vascularity and distinguish cysts from solid masses 1
• 94-100% sensitivity for detecting ovarian pathology 1

Specific Findings to Look For on Ultrasound:

• Simple or complex ovarian cysts that may be causing mid-cycle pain 1, 3
• Adenomyosis: thickened uterine wall with heterogeneous myometrium 2
• Endometriosis: endometriomas (chocolate cysts), nodularity, or fixed pelvic structures 2
• Hydrosalpinx or chronic inflammatory changes from subclinical infection 2, 3

When to Escalate Imaging

If ultrasound is nondiagnostic or inconclusive, MRI pelvis with gadolinium contrast is the problem-solving examination of choice for chronic pelvic pain. 1 MRI provides:

• Superior soft-tissue detail for detecting endometriosis, adenomyosis, and adhesive disease 1
• Direct visualization of pelvic varices if pelvic congestion syndrome is suspected 1
• Ability to distinguish inflammatory from neoplastic masses 1

Critical Red Flags Requiring Urgent Evaluation

Watch for these concerning features that change management:

• Any palpable mass requires urgent imaging and potential tissue diagnosis 3
• Unexplained vaginal bleeding mandates endometrial evaluation to exclude malignancy 3
• Acute severe pain with hemodynamic instability suggests ovarian torsion, ruptured cyst, or other surgical emergency 3
• Fever with pain raises concern for pelvic inflammatory disease or postpartum complications like ovarian vein thrombosis 4

Postpartum-Specific Considerations

In the postpartum period, additional diagnoses must be considered:

• Ovarian vein thrombosis occurs in 0.05-0.18% of pregnancies, typically presents with right lower quadrant pain, fever, and should be considered with lower abdominal pain and systemic symptoms 4
• Uterine complications from cesarean delivery, though these typically present earlier postpartum 5
• Chronic pelvic inflammatory disease can develop from instrumentation during delivery 1, 3

Common Diagnostic Pitfalls to Avoid

• Assuming gynecologic origin without systematic evaluation of gastrointestinal (inflammatory bowel disease, diverticulitis), urologic (interstitial cystitis), and musculoskeletal (pelvic girdle pain) causes leads to missed diagnoses 2, 3, 6
• Dismissing cyclic pain as "normal" without proper workup may miss serious pathology including ovarian neoplasm, which accounts for 8% of pelvic pain cases 1, 3
• Overlooking pelvic inflammatory disease when other etiologies are excluded, as PID can occur from recent delivery-related instrumentation 3

Practical Management Algorithm

1. Obtain detailed menstrual history: Confirm exact timing relative to menstrual cycle, duration of symptoms, and any changes since delivery 7
2. Perform pelvic examination: Assess for masses, adnexal tenderness, cervical motion tenderness, or uterine abnormalities 1, 3
3. Order transvaginal + transabdominal ultrasound with Doppler as first-line imaging 1
4. If ultrasound is negative or equivocal, proceed to MRI pelvis with contrast 1
5. Consider CT abdomen/pelvis with IV contrast only if broad differential includes gastrointestinal or urologic pathology 1