Being nauseous is usually an early sign of pregnancy. However, nausea as a sign and symptom of pregnancy rarely appears before you miss your period and before you have a positive pregnancy test.
If you are nauseous before you miss your period then you could be pregnant because nausea is among the typical pregnancy symptoms.
Nausea usually occurs because of an increase in the pregnancy hormone hCG, human chorionic gonadotropin. However, by the time your nausea appears as a sign of pregnancy you would expect to have a positive pregnancy test.
Only a positive pregnancy test will tell you whether you are pregnant or not. As you might already know, nausea is one sign of pregnancy but it's not among the earliest signs. The earliest signs of pregnancy are a missed period and a positive pregnancy test. Nausea as a typical pregnancy symptom rarely, if ever, happens until after you miss your period, 2-4 weeks after ovulation and fertilization.