Political intervention in reproduction has failed to raise the birth rate above the replacement level in the rich world, but has dramatically succeeded in lowering it below the replacement level in China (see One child policy).
A World government with a eugenic or small population policy could send humanity into 'voluntary' extinction. If developed world demographics are extrapolated they mathematically lead to 'soft' extinction before 3000 AD. (John Leslie estimates that if the reproduction rate drops to the German level the extinction date will be 2400).
Moreover, there are risks of infertility caused by hormonal disruption from the chemical/pharmaceutical industries, or biological changes, such as the (controversial) findings of falling sperm cell count in human males.
Humans could also simply evolve via traditional natural selection over a period of millions of years or by means of technology into a posthuman life-form or existence, leaving no trace of original Homo sapiens.