Praise be to Allah.
Firstly: wWe ask Allah to shower your wife with His mercy and to record for her the reward of martyrdom, because the Prophet (peace and blessings of Allah be upon him) said: “Martyrdom is of seven types besides being killed (in jihad) for the sake of Allah,” ...and among them he mentioned the woman who dies because of pregnancy or childbirth. Narrated by Abu Dawood, 3111; al-Nasai, 1846; classed as saheeh by al-Albani. And we ask Him to help you and her family to bear this loss with patience and to compensate you in this world and in the Hereafter.
Secondly: There is no doubt that life and death are in the hand of Allah and that the time of death and life-spans are decreed by His will. Allah says (interpretation of the meaning):
“Who has created death and life that He may test you which of you is best in deed”
“La ilaha illa Huwa (none has the right to be worshipped but He). It is He Who gives life and causes death”
No one has the power to benefit or harm another except by Allah’s leave. The Prophet (peace and blessings of Allah be upon him) said: “The Holy Spirit inspired to me that no soul will ever depart this world until it has completed its allotted time and used up its allotted provision, so do not be too desperate when seeking provision. If you feel that provision is slow in coming, do not try to hasten it by disobeying Allah, for that which is with Allah can only be attained by obeying Him.” Narrated by al-Tabarani, 8/166; classed as hasan by al-Albani).
Thirdly: Islam enjoins kindness. The Prophet (peace and blessings of Allah be upon him) said: “Allah has prescribed kindness in all one’s affairs.” Narrated by Muslim, 1955. The Prophet (peace and blessings of Allaah be upon him) even spoke of a woman who went to Hell because of a cat that she detained until it died. Narrated by al-Bukhari, 2365; Muslim, 2242.
To what degree then do you think Islam enjoins kindness towards one's wife– one’s companion in this world and in the Hereafter?
The Prophet (peace and blessings of Allah be upon him) said: “I urge you to treat women kindly.” Narrated by al-Bukhari, 3331; Muslim, 1468.
Al-Imam al-Tirmidhi narrated a hadeeth (1163) which he classed as hasan, and al-Albani agreed with him, according to which the Prophet (peace and blessings of Allah be upon him) said, during the Farewell Pilgrimage: “They (women) are like prisoners in your hands.”
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “The most complete in faith of the believers are those who have the best attitude, and the best of you are those who are best towards their womenfolk.” Narrated by al-Tirmidhi, 1162; classed as hasan by al-Albani.
Fourthly: If you know from the above that you fell short in your duty towards your wife, by cutting off communication with her when she was sick and was most in need of you, and the quarrel between you was about some minor issue, as you mention, then how could you fall short in spending on her, for spending on one’s wife is obligatory according to the Quran and Sunnah, and scholarly consensus, as it says in al-Mughni (9/229)? If your mother told you to do something other than that, you should not have obeyed her, because there is no obedience to any created being if it involves disobedience towards the Creator. Your feelings of shortcoming and sin are appropriate. The Prophet (peace and blessings of Allah be upon him) said: “Righteousness is a good attitude, and wrongdoing is that which wavers in your heart and which you dislike people finding out about.” Narrated by Muslim, 2553.
But this sin should not be regarded as the cause of your wife’s death, because it is not directly connected to it and such things do not usually kill a person. You mentioned in your question that she died because of complications in pregnancy. And you did well – may Allah reward you with good – by sitting with her in her last days and spending on her. Your doing these things will be an expiation for you in sha Allah.
Our advice to you is to pray a great deal for forgiveness for yourself and for her, and make du’a for her, give in charity on her behalf, honour her family and uphold your ties with them and treat them kindly; put up with any bad treatment you may encounter from them because of the loss of their daughter; and let what happened be a lesson for the future so that it does not happen again.
And Allah knows best.