I'm take phentermine 15mg for a short time. Can it cause negative effects on a fetus if taken during conception or early pregnancy?

Category C. Phentermine is a category C drug meaning there may be potential risk and it is only used if the benefits outweigh the risks. It is a weight loss drug. Consider starting birth control until you reach your target weight. The benefits of Phentermine are very limited.
Yes it can. and it is contraindicated during pregnancy, category X meaning it can cause fetal harm , please avoid during pregnancy or if planning to be pregnant.