Fifty percent of marriages among Muslims are reportedly between first cousins, rising to 70% in Pakistan and Saudi Arabia. This preference for cousin marriage is inspired by the fact that Prophet Muhammad married his first cousin, Zaynab bint Jahsh (RA) in 627 CE. Earlier, the Prophet (ﷺ) had suggested to Zaynab that she marry his adopted son and former slave, Zayd bin Haritha (RA). While Zaynab was apparently unhappy with that marriage proposal initially because she was a Qurayshi and he, a freed former slave, she subsequently acquiesced to that marriage when the following verse was revealed shortly thereafter to the prophet:
It is not fitting for a believer, man or woman, when a matter has been decided by Allah and His Messenger, to have any option about their decision. [Sûrah Al-Aḥzâb, 33:36]
But, apparently, theirs was not a happy marriage according to Martin Lings’ 2006 book, Muhammad: His Life Based on the Earliest Sources (p. 213).
This, then, is how the Prophet reportedly ended up marrying her:
”One day, when the Prophet wanted to speak to Zayd about something and went to his house, Zaynab opened the door, and she stood in the doorway telling him that Zayd was out but inviting him [in] none the less. A look passed between the two cousins which made each one conscious of a deep and lasting bond of love between them. In a moment the Prophet knew that Zaynab loved him and that he loved her and that she knew he loved her. The Prophet refused her invitation, and as he turned to go, she heard him say, ‘Glory to God the Infinite! Glory to Him who disposeth men’s hearts!’ When Zayd returned she told him of the Prophet’s visit and of the glorification she heard him utter. Zayd immediately went to him and said: ‘I have been told thou camest unto my home. Why didst [thou] not enter, thou who art more dear to me than my father and my mother? Was it that Zainab hath found favor with thee? If it is so, I will leave her.’ ‘Keep thy wife and fear God,’ said the Prophet with some insistence.” (Lings, 212).
Then the following verse was revealed:
Behold! You did say to one who had received the grace of Allah and your favor: “Retain (in wedlock) your wife and fear Allah.” But you hid in your heart that which Allah was about to make manifest: you feared the people, but it is more fitting that you should fear Allah. Then when Zayd had dissolved (his marriage) with her, with the necessary (formality), We joined her in marriage to you: in order that (in future) there may be no difficulty to the Believers in (the matter of) marriage with (former) wives of their adopted sons once the latter have dissolved with the necessary (formality) (their marriage) with them. [Sûrah Al-Aḥzâb, 33:37)
Since all actions of the Prophet—referred to collectively as sunnah— serve as models that Muslims desire to emulate, the above incident leading to the Prophet marrying his cousin is now considered a sunnah and often actively pursued as a preferred marital match. This practice of cousin marriage supposedly credits the couple and their respective parents with some extra merit, pertaining to rewards in the Hereafter.
O Prophet! We have made lawful to you your wives to whom you have paid their dowers; and those whom your right hand possesses out of the prisoners of war whom Allah has assigned to you; and daughters of your [paternal] uncles and aunts, and daughters of your maternal uncles and aunts, who migrated (from Mecca) with you; and any believing woman who dedicates her soul to the Prophet, if the Prophet wishes to wed her. This is only for you and not for the believers (at large); We know what We have appointed for them as to their wives and the captives whom their right hands possess, in order that there should be no difficulty for you. [Sûrah Al-Aḥzâb, 33:50]
So, marriages to all close relatives —including to “daughters of your uncles and aunts”— were permitted only to the prophet; these categories are what was not permitted to other Muslims.
Interestingly, and in spite of having this Divine permission to marry other closely related women, the Prophet subsequently married from various tribal lineages: He did not marry such closely related ladies after marrying Zaynab. For example, he did not marry Umm Habib bint al-Abbas (RA) (Ibn Ishaq, Al-Tabari, Ibn Sa’d), although she was not only his cousin, but also the daughter of his foster brother. And he declined the marriage proposal from two women: Durrah bint Abi Salama(RA) (Ṣaḥîḥ Muslim 3413) and Umama bint Hamza (RA) (Ibn Sa’d: Bewley/Saad 8:115-116)  because he discovered their fathers were his foster brothers.
Since giving as few as ten suckles to a baby by a wet nurse establishes a foster relationship between the wet nurse and the child (Al-Muwatta 30.12), foster relationships are relatively easy to acquire. Thus, based on verse 33:50 and backed up by verse 33:37, both cousin marriage and marrying children of foster parents are haram (forbidden) to the general Muslim population.
Accordingly, in the contemporary world, the question raised is this: What is so wrong with cousin marriage? Science provides us with a compelling reason: Cousin marriage increases chances of medical problems. With a higher amount of shared DNA, there is a higher risk of birth defects in babies born from such consanguineous marriages. Even in the absence of cousin marriages, the possibilities of genetic defects are higher in a population where there is a restricted social structure and thus a smaller genetic pool of eligible marriage prospects.
In Pakistan, where cousin marriage has been the norm for many generations, Prof. Anne-Marie Nybo Andersen (South Danish University), estimated that, compared to the five percent infant mortality prevailing in the general population, infant mortality among “doubled cousin” marriages (first cousin parents who were also children of first cousin parents) was nearly three times higher (13 percent). And it was nine percent among first cousins once removed, and seven percent among second cousins. Also, among the double first cousin progeny, 41 percent of pre-reproductive age (pre-teen?) deaths were associated with the expression of detrimental recessive genes, with equivalent values of 26, 15, and eight percent for first cousins; first cousins once removed/double second cousins; and second cousins, respectively (http://www.barrelstrength.com/2014/03/21/inbreeding/).
The most tragic report that I have read was an article by Izhar Ullah, Deformities in Charsadda: Cousin marriages, and the heavy price children pay, which appeared in the Karachi newspaper Dawn,(2015, October 15). In a village populated by descendants of two families for the past 40 years, almost every third home had children suffering from birth defects. Disabilities included blindness, cerebral palsy, mental disorders, thalassemia (a group of genetic diseases that affect the body’s hemoglobin), physical deformities, as well as hearing and speech impairments. And, while the residents were aware that offspring of inter-family marriages are likely to have birth defects or disabilities, they sadly continue with the practice due to “cultural norms.” This preferred practice, I suspect, goes back to a miscontrual of the lesson exemplified by the Prophet’s marriage with his first cousin, Zaynab.
A BBC report  discussing Pakistanis in the United Kingdom found that 55% of marriages were between first cousins. And many children came from repeat generations of first-cousin marriages. The report stated that these children were 13 times more likely than the general population to produce children with genetic disorders. They also found that one in ten children of first-cousin marriages in Birmingham in the U.K. either died in infancy or developed a serious disability.
The BBC report also stated that Pakistani-Britons, who account for only 3% of all births in the UK, produce about 30% of all British children with genetic illnesses. Studies show that the mean perinatal mortality in the Pakistani community of 16 per thousand significantly exceeded that in all other ethnic groups. Thus, congenital anomalies accounted for 41 percent of all British-Pakistani infant deaths.
Speaking against cousin marriage, the book Our Dialogue (Vol. 2, p. 308) 2 explains:
“The Islamic view is that while marriage between cousins is permitted, it is certainly preferable to choose a marriage partner from outside one’s family. We have to distinguish between what is permitted and what is advocated.”
Thus, Our Dialogue reports that the Prophet once advised a companion to choose for his son a wife from another tribe; and for his second son, a wife from still another. I would argue, however, that cousin marriage was permitted by the Divine only as an exception to the prophet; it was never meant to be for all Muslims.
In conclusion, the above summary underscores the inherent potential danger arising out of consanguineous marriage —a relationship that should be highly discouraged, not only because of health issues but also in conformity with understanding the directives given by Allah in Sûrah Al- Aḥzâb, 33:50. At a minimum, a blood test should be made mandatory to weed out potential problems arising out of such marriages.
So then, educated with this knowledge, will the majority of Muslims take heed and modify their preference for marriage matches?
 Ibn Sa’d: Kitâb Al-Tabaqat Al-Kabîr. Vol 8: The Women of Madina. Tr. Aisha Bewley. Ta Ha Publishers: London.