As early as how many months can the gender of the baby be determined with a sonogram?

After 3 months: A prenatal ultrasound scan can predict fairly accurately a baby's gender after 3 months of pregnancy. A prediction is just a prediction, so it is not 100% accurate. Unless parents have a genetic problem that either affects only boys or only girls, not knowing the gender until birth is very exciting! remember that ultrasounds were not even used until 1960's!

16 weeks: Possible after 16. For sure after 20 weeks.

Depends on Equipment: Our ultrasounds are reliable after 18 weeks if the fetus is in a good position. High resolution ultrasounds between 12-14 weeks when obtaining "nuchal thickness" screening can sometimes determine gender reliably. Remember it's never 100%, but usually accurate. It also helps to have an experienced sonographer and advanced training. Take care.