Final answer:
Using obstetric ultrasound between weeks 9 to 15 of pregnancy is the most accurate way to determine the biological sex of a fetus, achieving nearly 100 percent accuracy in sex estimation.
Step-by-step explanation:
The most accurate method to determine the biological sex of a fetus is through an obstetric ultrasound performed during weeks 9 to 15 of pregnancy. During this time, the fetus's reproductive organs develop rapidly and are clearly distinguishable by week 12, allowing for almost 100 percent accuracy in sex estimation. Previously, there have been cases where sex determination at birth led to incorrect identification and inappropriate surgical interventions, underlying the importance of accurate sex determination techniques.