Nausea and vomiting is very common in pregnant women. There are no certain reasons for morning sickness, but normally it occurs due to change in hormone level during the first trimester of pregnancy. It will occur any time in the day or night. If nausea and vomiting become severe, it can cause dehydration. It is called hyperemesis gravidarum, or hyperemesis. You may find weight loss due to it. You should consult your doctor in case of severe nausea and vomiting.