Is it true women with hyperemesis are having twins or are very likely to have twins?

Increased: Vomiting in pregnancy is associated with increased progesterone. If you are having twins you are more likely to have a stronger hormonal response, and more likely to vomitl.

Partly.: Each of these urban myths has a speck of truth. Hyperemesis gravidarum (hg) is due to the HCG which is proportionately increased with multifetal pregnancies; this means that women with multifetal pregnancies are more likely to suffer from hg compared to women carrying singleton pregnancies. Having said that, hg cannot be used to predict fetal plurality - prenatal ultrasound isd the test of choice.