Sa`id bin Musayyib and Abu Al-`Aliyah stated that the wisdom behind making the `Iddah of the widow four months and ten nights is that the womb might contain a fetus. When the woman waits for this period, it will become evident if she is pregnant. Similarly, there is a Hadith in the Two Sahihs narrated by Ibn Mas`ud stating:

«إنَّ خَلْقَ أَحَدِكُمْ يُجْمَعُ فِي بَطْنِ أُمِّهِ أَرْبَعِينَ يَوْمًا نُطْفَةً، ثُمَّ يَكُونُ عَلَقَةً مِثْلَ ذلِكَ، ثُمَّ يَكُونُ مُضْغَةً مِثْلَ ذلِكَ، ثُمَّ يُبْعَثُ إِلَيْهِ الْمَلَكُ فَيَنْفُخُ فِيهِ الرُّوح»

((The creation of) a human being is put together in the womb of his mother in forty days in the form of a seed, and next he becomes a clot of thick blood for a similar period, and next a morsel of flesh for a similar period. Then, Allah sends an angel who is ordered to breathe life unto the fetus.)

So, these are four months and ten more days to be sure, as some months are less (than thirty days), and the fetus will then start to show signs of life after the soul has been breathed into it. Allah knows best.