What indicates the commencement of fetal kidney function?

The presence of amniotic fluid is indicative of the commencement of fetal kidney function because the fetus begins to produce urine, which contributes to the overall volume of amniotic fluid. At around 12 weeks of gestation, the fetal kidneys start to develop and function more actively, leading to the production of urine that is excreted into the amniotic sac. This urine is a significant source of amniotic fluid, and the amount of fluid present can reflect the functional status of the fetal kidneys.

In contrast, the other options do not directly relate to the initiation of kidney function in the fetus. Decrease in maternal weight is more associated with gestational concerns rather than reflecting fetal kidney activity. Elevated levels of maternal alpha-fetoprotein can be associated with various conditions but are not specifically indicative of kidney function. Increased fetal movement may signify good overall fetal health but does not provide direct evidence of kidney activity. Thus, the presence of amniotic fluid is the key indicator of functional fetal kidneys.