Morning sickness is one of the common symptoms of pregnancy. It normally starts during third or fourth week of pregnancy and continue up to the first trimester. With start of second trimester or after 13 weeks, you can get relief from morning sickness. Many women believe that morning sickness can occur only during morning time, but it can occur any time in the evening or night. You can feel more during morning time when blood sugar level is down. Morning sickness cause the feeling of nausea and vomiting during initial stage of pregnancy. Sometimes it may also cause light headache in some women. Normally, morning sickness is a good sign and indicates that your pregnancy is in progress. Morning sickness is not harmful usually. It can be harmful only if vomiting occurs continuously, which may cause reduce in water level of the pregnant women. In such case, you should not wait longer and immediately contact doctor for taking right precautions.