Final answer:
Considering the presence of lanugo, developed nails, the experience of quickening, and fused eyes, the fetal demise most likely occurred around 22 weeks' gestation.
Step-by-step explanation:
The description of the fetal demise in this situation suggests that the fetus has characteristics that develop at different gestational ages. The presence of lanugo covering the body is typical in the second trimester, particularly between weeks 17-20, where it is most pronounced.
However, the presence of nails on fingers and toes and the feeling of quickening suggest a more advanced stage, closer to week 22, since nail formation on the digits is prominent and quickening is often felt around weeks 16-20.
The fact that the eyes are still fused indicates an age less than week 26, as after week 26, the eyelids would be developed enough to open and close. Therefore, based on the combined developmental milestones given, the fetus is most likely around 22 weeks' gestation.