Fetal sex is determined at conception - so the acronyms should be DFAC/DMAC... And nowadays fetal sex can also easily be ascertained at 8-9 weeks with NIPT, a pretty inexpensive form of genetic testing using the mother's blood drawn from her arm in the customary way. Which is well before the genitals would be developed enough to appear on a scan.

Fetal sex can also be ascertained at 8 weeks through CVS, a more costly and invasive procedure that's been in use for 30+ years. CVS involves a the mother getting a vaginal exam in which a needle/tube is inserted through the cervix and a piece of the placenta containing chorionic villi is taken, then sent to a genetics lab for analysis.

Later on in pregnancy, from 14-15 weeks, fetal sex prior to the era of the NPT could be ascertained through genetic testing of amnio fluid known as amniocentesis. Amniocentesis was first invented in the 1950s, and widely used from the 1960s-70s on in cases where genetic testing was recommended due to factors such as a history of genetic diseases in the mother and father's families and the mother being over 35.