I am 5 months pregnant and I was thinking about going on holiday to Vietnam, Thailand, or Bali. What is the risk of malaria in this countries?
Malaria transmission occurs in all of the nations you mention, though certain areas within these nations are lower risk.
You can find out more about specific regions and local antimalarial resistance patterns at the CDC website.
You should certainly consult your prenatal doctor to discuss the variety of potential risks, in addition to malaria, associated with international travel in the developing world as you enter the third trimester of pregnancy. Malaria infection in pregnant women can be more severe than in nonpregnant women and can increase the risk for adverse pregnancy outcomes. For these reasons, and because no chemoprophylactic regimen is completely effective, women who are pregnant are usually advised to avoid travel to areas with malaria transmission if possible. If travel to a malarious area cannot be deferred, chemoprophylaxis is essential. Chloroquine containing medications are have not been found to have harmful effects on the fetus. For pregnant women travelling to areas where chloroquine resistance is present, mefloquine is usually recommended.