The change in hormones during pregnancy affects everything. This includes processing of food and drink and smell. The morning sickness appears to be a warning that something is wrong with your body, and is trying to get rid of it from your body.

No one knows for sure, though there’s no shortage of theories. A few factors that may be at play include:

Increased levels of the pregnancy hormone hCG, which peaks around the time morning sickness is worst
Rising levels of estrogen and progesterone, which relax the muscles of the digestive tract and make digestion less efficient
The metallic taste that many women experience during pregnancy
A sharper sense of smell due to pregnancy hormones
Heartburn or gastrointestinal reflux disease (GERD), which are more common during pregnancy
Excess saliva, which can increase feelings of queasiness

Not all pregnant women experience morning sickness — and not in the same way. Some have only occasionally queasy moments. Others feel nauseous around the clock but rarely or never throw up. A few vomit frequently.

From what I’ve been told, the nausea is caused by all the new pregnancy hormones surging through your body