Pyelonephritis: This is a common cause of flank pain in pregnant women, especially during the second and third trimesters. The urinary tract changes during pregnancy, such as ureteral dilation, can predispose to infection.

Nephrolithiasis (Kidney Stones): Pregnancy increases the risk of developing kidney stones due to urinary stasis and increased calcium excretion. Flank pain is a classic symptom.
Musculoskeletal Strain: The growing uterus can cause a shift in the center of gravity, leading to muscle strain in the back and flank area.
Urinary Tract Infection (UTI): While not as severe as pyelonephritis, lower urinary tract infections can still cause significant discomfort and flank pain.

Placental Abruption: Although less common, placental abruption can cause severe abdominal and flank pain. It is a life-threatening condition for both the mother and the fetus.
Preeclampsia with HELLP Syndrome: This condition can cause right upper quadrant or flank pain due to liver involvement. It is a medical emergency.
Appendicitis: Although the appendix may be displaced upward during pregnancy, appendicitis can still occur and cause flank or abdominal pain. Delayed diagnosis can lead to severe consequences.

Renal Cell Carcinoma: Although rare, renal cell carcinoma can cause flank pain and should be considered in the differential diagnosis, especially if other symptoms like hematuria are present.
Ovarian Torsion: This is a rare condition that can cause severe flank pain. It is more common in pregnant women due to the displacement of the ovaries by the growing uterus.
Adrenal Hemorrhage: This is a rare but potentially life-threatening condition that can cause flank pain. It may be associated with trauma, severe hypertension, or coagulopathy.