**Core Concept**
The tympanogram is a graphical representation of the movement of the eardrum and the conduction of sound through the middle ear. It is used to assess the function of the middle ear system, particularly the eardrum, the ossicles, and the tympanic membrane. There are four main types of tympanograms: A, Ad, As, and B.

**Why the Correct Answer is Right**
The patient's symptoms of bilateral hearing loss that worsened during pregnancy are suggestive of Eustachian tube dysfunction. The Eustachian tube connects the middle ear to the nasopharynx and plays a crucial role in equalizing the air pressure in the middle ear. During pregnancy, the increased nasal congestion and Eustachian tube dysfunction can lead to middle ear effusion and hearing loss. An As tympanogram is characteristic of Eustachian tube dysfunction, indicating a flat tympanogram with a low compliance. This is because the eardrum is fixed in place due to the middle ear effusion, resulting in a lack of movement and a low compliance.

**Why Each Wrong Option is Incorrect**
**Option A:** An Ad tympanogram is characteristic of ossicular discontinuity or fixation, which is not consistent with the patient's symptoms of Eustachian tube dysfunction.
**Option C:** A C tympanogram is characteristic of a normal middle ear system, which is not consistent with the patient's symptoms of hearing loss.
**Option D:** A B tympanogram is characteristic of a tympanic membrane perforation, which is not consistent with the patient's symptoms of Eustachian tube dysfunction.

**Clinical Pearl / High-Yield Fact**
Eustachian tube dysfunction is a common cause of middle ear effusion and hearing loss, particularly in pregnant women. The symptoms can be relieved with nasal decongestants and Eustachian tube exercises, such as swallowing, yawning, or blowing the nose gently.

**✓ Correct Answer: B. As**