Pyelonephritis: This is a common condition in pregnancy due to urinary stasis and ureteral dilation, which can lead to bacterial infection of the kidney. The symptoms include flank pain, fever, and urinary frequency.

Nephrolithiasis (Kidney Stones): Pregnancy increases the risk of kidney stones due to increased urine calcium excretion and urinary stasis. Symptoms include severe flank pain, nausea, vomiting, and sometimes hematuria.
Musculoskeletal Strain: The growing uterus can cause a shift in the woman's center of gravity, leading to strain on the back muscles, which may cause flank pain.
Urinary Tract Infection (UTI): While not as severe as pyelonephritis, UTIs are common in pregnancy and can cause flank pain, dysuria, and frequent urination.

Placental Abruption: Although less common, placental abruption can cause severe abdominal or back pain and is a life-threatening condition for both the mother and the fetus if not promptly diagnosed and treated.
Preeclampsia with HELLP Syndrome: This condition can cause right upper quadrant or epigastric pain but can sometimes radiate to the flanks. It's a serious complication of pregnancy that requires immediate medical attention.
Ovarian Torsion: Although rare in pregnancy, ovarian torsion is an emergency that requires prompt surgical intervention to save the ovary.

Renal Cell Carcinoma: Although rare, any new onset of flank pain in pregnancy should consider the possibility of renal cell carcinoma, especially if associated with hematuria or a palpable mass.
Adrenal Gland Disorders: Disorders such as pheochromocytoma or adrenal hemorrhage are rare but can cause flank pain and are critical to diagnose due to their potential impact on both mother and fetus.