Ovarian Hyperstimulation Syndrome (OHSS): This condition is a common complication of fertility treatments, which can cause bilateral adnexal masses due to ovarian enlargement. The size of the masses (10 cm) and their bilateral nature, especially in the context of pregnancy, make OHSS a highly plausible diagnosis.

Ovarian Cysts: Large ovarian cysts can occur in pregnancy and may be bilateral. They can be simple or complex and may cause adnexal masses.
Theca Lutein Cysts: Associated with gestational trophoblastic disease or hyperreactio luteinalis, these cysts can cause significant ovarian enlargement and are more likely in the context of high hCG levels, such as in a molar pregnancy or multiple gestations.
Dermoid Cysts: Although less common to present bilaterally and as large as 10 cm, dermoid cysts (mature cystic teratomas) can grow to significant sizes and may be discovered during pregnancy.

Ovarian Torsion: Although not a primary cause of bilateral adnexal masses, ovarian torsion is a surgical emergency that can occur in the presence of large ovarian masses. Missing this diagnosis could lead to loss of the ovary and potentially compromise the pregnancy.
Malignant Ovarian Neoplasms: While less common, malignant ovarian tumors can present as large adnexal masses during pregnancy. The consequences of missing this diagnosis could be severe, making it crucial to consider.

Lymphoma: Rarely, lymphoma can involve the ovaries and present as large masses. This would be an unusual presentation but is a consideration in the differential diagnosis.
Metastatic Disease to the Ovaries: In the context of a known primary malignancy, metastases to the ovaries could present as bilateral adnexal masses, although this is rare in pregnancy.