Final answer:
A fetus can usually be detected on a pelvic ultrasound around 6-7 weeks gestation, with the reproductive organs developing rapidly between 9 to 15 weeks. By week 12, the biological sex of the fetus can be determined with almost 100 percent accuracy.
Step-by-step explanation:
On a pelvic ultrasound, a fetus can typically be detected around 6-7 weeks gestation. However, the timing may vary depending on the skill of the sonographer and the quality of the equipment used. At around 9 to 15 weeks gestation, the reproductive organs of the fetus start developing rapidly, and by week 12, the biological sex of the fetus can be determined with almost 100 percent accuracy using obstetric ultrasound.