Symptoms of nausea in the afternoon are not necessarily caused by pregnancy. After a woman becomes pregnant, usually after 40 days or more of pregnancy, she may experience certain pregnancy reactions, including nausea, vomiting and dizziness. However, there are many causes of nausea, and pregnancy is not the only cause of nausea in women. For example, problems with the digestive system or other causes may cause nausea in women, so you cannot tell if you are pregnant by experiencing nausea. Whether or not a woman is pregnant is determined by testing for human chorionic gonadotropin (HCG) in the blood or HCG in the urine with a pregnancy test, so nausea alone is not the only way to tell if you are pregnant.