The sex of a foetus is determined during what stage?

Gender: Gender is determined at conception.

Depends.: Your doctor can tell you the fetus' gender either by looking at the apparent external genitalia on imaging (u/s, mri), or by dna analysis (xx vs. Xy). In the first case, we could tell fetal sex by 10 weeks with some degree of uncertainty (<10%). In the second case, new non-invasive prenatal diagnosis methods are now available to test this after 7 weeks of pregnancy (maternit21) with >99% accuracy.