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

Nephrolithiasis (Kidney Stones): Although less common in pregnancy, kidney stones can cause severe flank pain. The pain can radiate to the groin and is often accompanied by nausea, vomiting, and urinary frequency.
Musculoskeletal Strain: The growing uterus can put strain on the back muscles, leading to musculoskeletal pain that can be felt in the flank area.
Urinary Tract Infection (UTI): While not as severe as pyelonephritis, a lower urinary tract infection can still cause discomfort and pain in the flank area.

Placental Abruption: Although more commonly associated with vaginal bleeding, placental abruption can cause severe abdominal and flank pain. It is a life-threatening condition for both the mother and the fetus.
Preeclampsia: This condition can cause severe headaches, vision changes, and abdominal pain, including flank pain. It is a serious condition that requires immediate medical attention.
Ovarian Torsion: Although rare in pregnancy, ovarian torsion is a surgical emergency that can cause severe, sudden-onset flank pain.

Renal Cell Carcinoma: A rare cause of flank pain, renal cell carcinoma can occur during pregnancy, although it is not commonly associated with pregnancy itself.
Adrenal Gland Disorders: Such as pheochromocytoma or adrenal hemorrhage, these conditions are rare but can cause flank pain and are important to consider in the differential diagnosis.