Final answer:
Fetal sex can be accurately determined by ultrasound during weeks 9 to 15 of pregnancy.
Step-by-step explanation:
The sex of a fetus can be determined by ultrasound with nearly 100 percent accuracy between weeks 9 to 15 of pregnancy, as the fetus's reproductive organs rapidly develop during this period. Although male and female external genitalia are quite similar in appearance initially, they will be clearly differentiated by week 12. Advanced ultrasound systems, including 3D imaging, can reveal remarkable detail and are commonly used in prenatal care to monitor the development of the fetus and help in determining sex as well as identifying any potential abnormalities early on.