It is estimated that 50% to 90% of pregnant women experience morning sickness or a more severe condition, hyperemesis gravidarum, in which women vomit several times a day. Morning sickness typically will start between the 4th and 6th weeks of pregnancy, and should typically subside by the 14th or 16th week.
The nausea and vomiting that often occur during morning sickness will typically not threaten your baby in any way. As long as you are able to keep some food down and are able to keep up on fluids, your teenie tiny little one doesn't require very many calories and should be just fine. If you find that you're unable to keep down enough food you should make sure to take a multivitamin to make sure you're getting the nutrients that you and the baby need. During this time, you should use a multivitamin that is low in iron, as iron can make your nausea worse.
Dehydration during pregnancy is always a risk if you have severe vomiting. Due to this risk you should make sure that you are getting plenty of fluids.