What weeks can one determine the baby sex?

After 20. An ultrasound after 20 weeks can generally determine the sex of the baby with high degree of certainty.
Depends. Some ultrasound technicians can spot features around 16 weeks but few are ready to call it until a few weeks later.There is an interesting maternal blood test that can examine fetal cell chromosomes and determine gender (or paternity) in the first trimester.It is a bit pricey.