Cause of morning sickness

As you become pregnant you get yourself prepare for the arrival of a new baby.There are many physical changes that take place in your body.These changes may let the women feel nausea or vomiting and probably a combination of all these causes morning sickness.
There are many reasons that leads to morning sickness in the women. They are like

The rising levels of the hormones human chorionic gonadotropin(hCG) and estrogen during the same time that nausea commonly occurs are prime suspects though there is no confirmed proof of this finding

Also during pregnancy women get very sensitive about smell. Thus any strong odor triggers a nauseating feeling probably due to a high level of estrogen.

Eating greasy and spicy foods are also regarded as morning sickness causes. As the digestion deteriorates under the influence of hormone levels, consumption of these foods aggravates the situation and makes pregnant women more susceptible to morning sickness.

In case you are carrying twins your body produces twice the number of hormones so the chances of morning sickness increases more.Also it may be hereditary.
There are many more other reasons too.