Does being pregnant with twins cause extreme nausea?

Yes. Hyperemesis gravidarum (severe nausea and vomiting during pregnancy) having a multiple pregnancy is one of the risk factors of this disease. The other risk factors are: hyperemesis gravidarum during a previous pregnancy, being overweight, being a first-time mother, the presence of trophoblastic disease, which involves the abnormal growth of cells inside a woman's uterus.

Yes. Double the hormones, double the nausea.