Pulmonary Embolism (PE): Given the symptoms of shortness of breath, racing heart, and feeling faint in a pregnant patient, PE is a highly plausible diagnosis due to the increased risk of thromboembolic events during pregnancy.

Anxiety or Panic Attack: These conditions can present with similar symptoms and are common in pregnancy due to hormonal changes and stress.
Asthma or Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: Pre-existing respiratory conditions can worsen during pregnancy, leading to symptoms like shortness of breath and a racing heart.
Cardiac Conditions (e.g., Peripartum Cardiomyopathy): Pregnancy can unmask or exacerbate underlying heart conditions, leading to symptoms such as shortness of breath and palpitations.

Pulmonary Embolism (PE): Although listed as the single most likely diagnosis, it's crucial to reiterate its importance as a "do not miss" diagnosis due to its high mortality rate if untreated.
Amniotic Fluid Embolism: A rare but potentially fatal condition that can occur during pregnancy, characterized by the sudden onset of symptoms like shortness of breath and cardiovascular collapse.
Placental Abruption: A serious condition where the placenta separates from the uterus, which can cause maternal hypovolemia and symptoms like fainting and shortness of breath.
Uterine Rupture: Although more commonly associated with abdominal pain, uterine rupture can lead to maternal shock and symptoms like fainting and shortness of breath.

Pregnancy-Related Spontaneous Coronary Artery Dissection (SCAD): A rare condition that can cause cardiac symptoms similar to those described.
Idiopathic Pulmonary Arterial Hypertension: A rare condition that can present with shortness of breath and may be exacerbated by pregnancy.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, which can present with symptoms like palpitations, shortness of breath, and fainting.