Costochondritis: This condition, also known as Tietze's syndrome, is characterized by inflammation of the cartilage that connects the ribs to the breastbone. It is a common cause of chest pain and can be exacerbated by the physical changes of pregnancy, such as increased breast size and rib cage expansion.

Musculoskeletal strain: The growing uterus and changes in posture during pregnancy can put strain on the muscles and joints of the chest wall, leading to pain under the left breast.
Gastroesophageal reflux disease (GERD): Pregnancy can exacerbate GERD symptoms, which may include chest pain or discomfort under the breast.
Breast tenderness: Hormonal changes during pregnancy can cause breast tenderness or pain, which may be localized under the left breast.

Pulmonary embolism: Although less common, pulmonary embolism is a life-threatening condition that requires immediate attention. Chest pain, particularly if it is severe, sudden, or accompanied by shortness of breath, should prompt consideration of this diagnosis.
Myocardial infarction: Pregnancy increases the risk of myocardial infarction, and chest pain under the left breast could be a symptom. It is crucial to consider this diagnosis, especially in women with risk factors for cardiovascular disease.
Pneumonia: Infection can cause chest pain, and pneumonia should be considered, especially if accompanied by fever, cough, or shortness of breath.

Pneumothorax: A collapsed lung can cause sudden, severe chest pain and is more common in pregnant women due to the increased pressure on the lungs.
Diaphragmatic hernia: Although rare, a diaphragmatic hernia can cause chest pain and should be considered if there are symptoms of bowel obstruction or if the pain is accompanied by abdominal symptoms.
Thoracic spine issues: Conditions such as thoracic disc herniation or spinal stenosis can cause chest pain and should be considered if there are associated neurological symptoms or if the pain is exacerbated by movement.