In pregnancy anti allergic in 3rd trimester like cetirizine or levocetrizine-is it safe?

This may be caused by an increase in estrogen and will go away once she delivers, BUT severe itchiness in the second or, more commonly, the third trimester can be a sign of intrahepatic cholestasis of pregnancy (ICP) or obstetric cholestasis, a liver problem that affects up to 1 percent of pregnant women in the United States.
When bile doesn't flow normally in the small ducts of the liver, bile salts accumulate in the skin, which makes her itchy. The itching may affect her soles and palms first, but it can occur anywhere, and many women with ICP feel itchy all over. The itchiness can be extremely intense and is typically worse at night.
Itching is the only symptom at first. The condition itself doesn't cause a rash, but she may end up with red, irritated skin with little cuts in areas where she is scratching a lot. Some women have other symptoms, too, such as loss of appetite, mild nausea, and malaise. A minority of women develop mild jaundice. She needs blood work to check her liver function looking for elevated LFT results as well as serum bile acid counts, and an ultrasound to check the baby for fetal distress. ICP can lead to still birth so this is not something to ignore. She should be tested. If she does have the condition, it is recommended to deliver at 37-38 weeks. To obtain a diagnosis of ICP, there are two LFT and Serum bile acid test. The liver function tests (LFTs) is a simple blood test, the results of which should be available by the next day. If the ALT level is elevated, this, plus pruritus of palms and soles, should be treated as diagnostic of ICP (however LFT's are not always elevated in ICP patients). The serum bile acid blood test for ICP is a quantitative measurement of bile salts. The results of this test often take longer to return, but the test is more specific for ICP. Upon diagnosis, most providers will prescribe Ursodeoxycholic Acid. While there is no cure for ICP, and no way to guarantee a successful outcome, studies have shown a slightly better fetal and maternal outcome from administration of Ursodeoxycholic Acid, whereas Cholestyramine appears to only relieve itching.
If additional blood tests to check clotting function identify a problem, giving Vitamin K may help avoid the risk of hemorrhage at delivery.
Delivery by 37 completed weeks is considered crucial to fetal outcome, though it does not completely eliminate the risks associated with the condition.

Cetirizine has been assigned to pregnancy category B by the FDA. High dose animal studies have failed to reveal evidence of teratogenicity. There are no controlled data in human pregnancy. Cetirizine is only recommended for use during pregnancy when need benefit outweighs risk. You could try it but if she wont likely respond if she has ICP and she should be tested to rule ICP out.