There are many miracle of Quran which prove that it is Allah's words. Some of the miracles are:
Human Embryonic Development
In the Holy Quran, God speaks about the stages of man’s embryonic development:
وَلَقَدْ خَلَقْنَا الْإِنسَانَ مِن
سُلَالَةٍ مِّن طِينٍ (O) ثُمَّ جَعَلْنَاهُ نُطْفَةً فِي قَرَارٍ مَّكِينٍ (O) ثُمَّ
خَلَقْنَا النُّطْفَةَ عَلَقَةً فَخَلَقْنَا الْعَلَقَةَ مُضْغَةً فَخَلَقْنَا
الْمُضْغَةَ عِظَاماً فَكَسَوْنَا الْعِظَامَ لَحْماً ثُمَّ أَنشَأْنَاهُ خَلْقاً
آخَرَ فَتَبَارَكَ اللَّهُ أَحْسَنُ الْخَالِقِينَ (O)
"Man We did create from a quintessence (of clay); Then We placed him as (a drop of) sperm in a place of rest, firmly fixed; Then We made the sperm into a clot of congealed blood; then of that clot We made a (foetus) lump; then we made out of that lump bones and clothed the bones with flesh; then we developed out of it another creature. So blessed be Allah, the best to create!" (Al-Mumenoon: 12-14)
Literally, the Arabic word alaqah has three meanings: (1) leech, (2) suspended thing, and (3) blood clot.