Level 1: Causes of nasal congestion in pregnant women. The cause of nasal congestion in pregnant women may be caused by viral infection or allergies. Viral infections often cause the lining of the nose to swell, making it difficult to breathe. Allergies, on the other hand, can cause the lining of the nose to overreact, causing congestion and swelling. In addition, pregnant women are also susceptible to viral and bacterial infections due to their weakened immunity. Level 2: Dangers of nasal congestion during colds during pregnancy Pregnant women have colds