The reason why the ‘iddah of a woman whose husband dies is four months and ten days
Question

What is the reason why a woman’s ‘iddah is four months and ten days if her husband dies?.
Praise be to Allah.There are two ways to know the reason behind the command of Allah or the command of His Messenger (peace and blessings of Allah be upon him). 
1 – The reason may be stated in a text of the Quran or Sunnah, such as the verses in which Allah says (interpretation of the meaning): 
 
“And We made the Qiblah (prayer direction towards Jerusalem) which you used to face, only to test those who followed the Messenger (Muhammad) from those who would turn on their heels (i.e. disobey the Messenger)” [al-Baqarah 2:143]
“Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allah after the (coming of) Messengers. And Allah is Ever All‑Powerful, All‑Wise” [al-Nisa’ 4:165]
And such as the words of the Prophet (peace and blessings of Allah be upon him): “Visit the graves, for they remind you of death.” Narrated by Muslim (976). 
There are many such texts in which the reasons for commands are stated. 
2 – The scholars may derive the reasons by means of study and ijtihaad, and they may be right or wrong, and the reason may be concealed from many people. What is required of the believer is to submit to the command of Allah and obey it in all cases, whilst firmly believing that Allah is Wise and has great wisdom, and He is not to be questioned about what He does, whereas they will be questioned about what they do. 
Secondly: 
Allah has commanded women to observe an ‘iddah of four months and ten days in the event of the husband’s death. He says (interpretation of the meaning): 
“And those of you who die and leave wives behind them, they (the wives) shall wait (as regards their marriage) for four months and ten days, then when they have fulfilled their term, there is no sin on you if they (the wives) dispose of themselves in a just and honourable manner (i.e. they can marry). And Allah is Well-Acquainted with what you do” [al-Baqarah 2:234]
Allah did not clearly state the reason for that, but the scholars worked out what they thought was the reason, based on the general principles of Islam with regard to protecting lineages and honour. 
Ibn Katheer (may Allah have mercy on him) said in his Tafseer: Sa’eed ibn al-Musayyib, Abu’l-‘Aaliyah and others stated that the reason why the ‘iddah in the case of the husband’s death is four months and ten days is because of the possibility of pregnancy. If (the woman) waits for this length of time, it will become apparent whether she is pregnant, as is mentioned in the hadeeth of Ibn Mas’ood which is narrated in al-Saheehayn and elsewhere: “The creation of any one of you is put together in his mother’s womb as a nutfah (drop) for forty days, then he becomes a ‘alaqah (a piece of thick coagulated blood) for a similar period, then he becomes like a chewed piece of flesh for a similar period, then Allah sends an angel to him and he breathes the soul into him.” These three periods of forty days add up to four months, and to be on the safe side ten days are added to that, because some months may be shorter, and because the movements of the foetus begin after the soul is breathed into it. And Allah knows best. 
Sa’eed ibn Abi ‘Aroobah said, narrating from Qatadah: I asked Sa’eed ibn al-Musayyib: What is it about the ten days? He said: At that time the soul is breathed into (the foetus). Al-Rabee’ ibn Anas said: I said to Abu’l-‘Aaliyah: Why are these ten days added to the four months? He said: Because at that time the soul is breathed into (the foetus). Both reports were narrated by Ibn Jareer. End quote. 
Al-Shawkaani (may Allah have mercy on him) said in Fath al-Qadeer: The reason why the ‘iddah in the case of the husband’s death is this length of time is that the male foetus usually begins to move after three months, and the female after four, and Allah added ten days to that because the foetus may be weak and its movements may be delayed somewhat, or not. End quote. 
See: Zaad al-Maseer by Ibn al-Jawzi (1/275) and I’laam al-Muwaqqi’een (2/52). 
It should noted that it is not permissible to ignore a shar’i ruling on the basis that there may be an alternative way of achieving the purpose behind it. No one has the right to say: “If the reason for the ‘iddah is to establish whether the woman is pregnant or not, modern medicine enables us to know that at the beginning of pregnancy, so there is no need for a woman to wait for this length of time.” He does to have the right to say that because the reason mentioned is something that the scholars have worked out, but it may be wrong, or it may be only part of the reason and not the entire reason. So it is not permissible to ignore the definitive command on which there is consensus, for a derived reason which may be wrong. 
And Allah knows best.