Final answer:
The biological sex of a fetus can usually be determined with almost 100 percent accuracy using obstetric ultrasound at around week 12 of pregnancy.
Step-by-step explanation:
The biological sex of a fetus can usually be determined with almost 100 percent accuracy using obstetric ultrasound at around week 12 of pregnancy. During weeks 9 to 15 of pregnancy, the reproductive organs of the fetus develop rapidly, and by week 12, the external genitals of male and female fetuses are clearly differentiated.