Animation of the male genitalia of a 12-week fetus. The genitalia of the fetus develop from week 4 and becomes external from week 8. Up until this point, the embryonic sex organs in boys and girls are identical. Whether a girl or boy will be born is determined by the steps that occur in the next few weeks. A boy is born when the urethral groove closes and the initial protrusion shapes to become a penis. There are particles floating suggesting the fetus is in utero.