Normal Pregnancy-Related Edema: This is the most common cause of pedal edema in a pregnant woman at 40 weeks gestation. The increased fluid volume, venous pressure, and decreased mobility contribute to fluid accumulation in the lower extremities.

Preeclampsia: A condition characterized by high blood pressure and often accompanied by proteinuria. It can cause edema, but it's typically more severe and associated with other symptoms like headaches and vision changes.
Varicose Veins: Pregnancy increases the risk of varicose veins due to increased venous pressure. These can cause edema, pain, and discomfort in the legs.
Deep Vein Thrombosis (DVT): Although less common, DVT should be considered, especially if there's a history of thrombophilia or recent immobilization.

Severe Preeclampsia or Eclampsia: These conditions can lead to significant morbidity and mortality for both the mother and the fetus if not promptly recognized and treated.
Pulmonary Embolism: A potentially life-threatening condition that can occur in conjunction with DVT. Symptoms might include sudden onset of shortness of breath, chest pain, and tachycardia.
Heart Failure: Pre-existing heart conditions can decompensate during pregnancy due to the increased fluid volume and cardiac workload.

Nephrotic Syndrome: A condition characterized by severe proteinuria, hypoalbuminemia, and edema. It's rare in pregnancy but can be associated with preeclampsia or other renal diseases.
Thyroid Disorders: Both hypothyroidism and hyperthyroidism can cause edema, though they are less common causes in the context of pregnancy-related pedal edema.
Lymphatic Obstruction: Rare conditions like lymphoma or other malignancies can cause lymphatic obstruction leading to edema, but these are exceedingly rare in the context of pregnancy.