Urticaria of pregnancy: This is the most common cause of hives in pregnancy, accounting for approximately 70% of cases. It is thought to be related to the hormonal changes and immune system modulation that occur during pregnancy.

Allergic reactions: Allergic reactions to foods, medications, or environmental factors can cause hives in pregnancy, just as they can in the non-pregnant population.
Infections: Infections such as urinary tract infections, respiratory tract infections, or skin infections can trigger hives in pregnant women.
Autoimmune disorders: Conditions like thyroid disease or lupus can cause hives in pregnancy, although these are less common.

Preeclampsia: Although rare, preeclampsia can present with hives, and it is a potentially life-threatening condition for both the mother and the fetus if not promptly recognized and treated.
HELLP syndrome: A variant of preeclampsia, HELLP syndrome (Hemolysis, Elevated Liver enzymes, and Low Platelet count) can also present with hives and requires immediate medical attention.
Anaphylaxis: A severe, life-threatening allergic reaction that can cause hives, among other symptoms. Prompt recognition and treatment with epinephrine are crucial.

Hereditary angioedema: A rare genetic disorder that can cause recurrent episodes of hives, often without an identifiable allergen or cause.
Mastocytosis: A condition characterized by an accumulation of mast cells in the skin, which can cause hives and other symptoms.
Cholestasis of pregnancy: A condition that occurs in some pregnant women, characterized by severe itching (pruritus) and can sometimes present with hives, although this is less common.