I believe you are worried about the itching when you mention allergy.
Itching in this condition is not due to allergy , its due to the elevated bile acids in blood as a complication of liver dysfunction
She is having Intahepatic cholestatsis of pregnancy.( obstetric cholestasis
The cause of this disease is not completely understood , but your wife require urgent care by a competent obstetrician.
Drugs that can be of help include ursodeoxycholic
acid and cholestyramine
It resolves after delivery. If the pregnancy is past 37 weeks , discuss with your doctor regarding early delivery
Clotting function , mothers blood pressure and babys heart rate
will need to monitored.
Also ensure that infective causes of hepatitis
like A,E,B,C are ruled out
Wishing all the best for your wife and your baby