Differential Diagnosis for Severe Loin Pain in a 26-Week Pregnant Woman

Single Most Likely Diagnosis

• Nephrolithiasis (Kidney Stones): This condition is a common cause of severe loin pain, especially in pregnant women due to physiological changes that increase the risk of stone formation. The pain is typically colicky and can radiate to the groin.

Other Likely Diagnoses

• Pyelonephritis: An infection of the kidney, which can cause severe loin pain, but would typically be accompanied by other symptoms such as fever and urinary symptoms. However, in early stages or in pregnancy, some symptoms might be masked or atypical.
• Musculoskeletal Pain: Pregnancy can exacerbate musculoskeletal issues due to weight gain and posture changes, leading to loin pain. However, this would typically be more diffuse and less severe than the pain caused by kidney stones or infection.
• Urinary Tract Infection (UTI): While a normal urine dip does not rule out a UTI entirely, it makes it less likely. However, UTIs can cause significant pain and should be considered, especially if there are other symptoms like dysuria or frequency.

Do Not Miss Diagnoses

• Placental Abruption: Although more commonly associated with vaginal bleeding, placental abruption can cause severe abdominal or back pain and is a life-threatening condition for both the mother and the fetus.
• Preeclampsia with Posterior Reversible Encephalopathy Syndrome (PRES) or HELLP Syndrome: Despite normal blood pressure, it's crucial to consider preeclampsia and its complications, as they can present atypically. Severe headache, vision changes, or right upper quadrant pain might accompany loin pain in these conditions.
• Ovarian Torsion: Although rare, ovarian torsion can cause severe, sudden-onset pain and is a surgical emergency.

Rare Diagnoses

• Renal Infarction: A rare condition where the blood supply to the kidney is interrupted, causing severe pain. This would be unusual in a young, otherwise healthy pregnant woman without risk factors for thrombosis.
• Retroperitoneal Hemorrhage: Bleeding into the space behind the peritoneum can cause severe back or loin pain and is a medical emergency. This could be related to trauma, anticoagulation, or vascular anomalies.