Question 36: Why are foster siblings (milk siblings) not allowed to marry? Is there a scientific study about milk kinship?

The Answer

Dear Brother / Sister,

Answer: The religion of Islam prohibits the marriage of foster siblings. There are certainly some reasons for it. However, it is necessary to obey that command since Allah orders it whether we know those reasons or not.

It is clearly stated in the Quran what people a Muslim cannot marry. Islam regards no difference between “giving birth” and “breastfeeding” in terms of marriage prohibition. It is haram to marry one’s wet nurse; it is also haram for milk siblings to marry one another. 

The prohibition related to milk kinship is formulated as follows in Islam: “The person who is breastfed is haram to the one who breastfeeds him. The offspring of the women who breastfeeds is haram to those whom she breastfeeds.” 

This decree is expressed as follows in the Quran:

“Prohibited to you (For marriage) are:- Your mothers, daughters, sisters; father´s sisters, Mother´s sisters; brother´s daughters, sister´s daughters; foster-mothers (Who gave you suck), foster-sisters; your wives´ mothers; your step-daughters under your guardianship, born of your wives to whom ye have gone in,- no prohibition if ye have not gone in;- (Those who have been) wives of your sons proceeding from your loins; and two sisters in wedlock at one and the same time, except for what is past; for Allah is Oft-forgiving, Most Merciful”(an-Nisa, 23)

What is the content of human breast milk?1

Human breast milk is divided into three: colostrum, transitional breast milk and mature breast milk.


It is the first milk given to the baby in the first 4-5 days after birth.It is very rich in nutrients. There are abundant lactoferrin and bifidum factors, which are very important for the child in colostrum. Its composition differs from normal milk; it is yellowish, slightly salty and sweet. It contains high levels of protein and immunizing immunoglobulin. They undergo coagulation when they are boiled because they contain high levels of serum protein and high pH.

A substance called lactoferrin in breast milk prevents the reproduction of microbes in the intestines and provides easier absorption of the iron necessary for the body. There are proteins, especially bioactive proteins, peptides, amino acids, carbohydrates, fats, minerals and antibodies in breast milk.

Amino acids guarantee the activities of all enzymes and immune factors in human organism, activate the metabolism of cells, promote development, mental strength and fitness.

Especially the proteins in breast milk are genome products of the mother and affect the body development, anatomy, physiology and behavior profiles of the baby. The word genome expresses the entire genetic program placed in the cells of any living being.

It is seen that when a mother breastfeeds two different babies, the babies have the same genome stimuli and they become biologically and emotionally similar to the woman breastfeeding them.

There are many biologically active molecules in breast milk. The most important of them are miRNAs, will affect gene regulation in the baby, similar structures and antibodies.

Antibodies are proteins prepared as an answer to an antigen stimulus. These proteins have a globulin structure. They are called “immuneglobulins” since they take part in immunity incidents in body defense. These are shown as “Ig”. They protect the body against foreign, harmful enemies and antigens. Antibodies are produced by plasma cells. There are five types of immune globulins in man. They are IgG is IgA, IgM, IgD and IgE.

To summarize:

1. Breast milk includes interleukin, lactoferrin, lysozyme and high IgA content.

2. Newborn babies are protected from infections by the placenta before birth and by antibodies that they receive with breast milk after birth until their immune system develops. Only IgG out of the three main immunoglobulins (IgG, IgA, IgM) can cross placental barrier. This provides immunity to specific viral infections. At birth, the baby’s IgG level is equal to or slightly higher than that of the mother.

3. Immune cells are also abundant in breast milk. They are leukocytes (white blood cells). They are mostly found in colostrum. They fight foreign substances that enter the body and germs. They continue acting as phagocytes in the intestines of infants. That is, they absorb and digest a substance in the outer environment of the cell.

Macrophages are also abundant though not as many as immune cells.  Macrophages are part of the innate immune system. They are cells responsible for the ingestion of dead cells, cellular debris and foreign substances in the body.

3. Human milk also has miRNAs, which are also called “mirna” andwhich are small non-coding RNAs.

The main task assigned to them is to arrange gene expressions at the post-transcriptional level. Human milk is very rich in mirnas. These mirnaspass from mother to baby after breastfeeding. They enable the regulation of many genes related to development in the baby. Besides, it has been found in recent studies that microRNAs play a role in the regulation of genes related to pluripotency. The importance of it is that it can reprogram somatic cells and transform them into pluripotent stem cells. Thus, stem cell profiles can change as babies are breastfed.

The meaning of pluripotency or pluripotent is undifferentiated cells that exist in the early stages of a developing embryo and that are capable of being transformed into all living cell types.

These changes take place in three stages:

1-In this first step, the 5’ end of the RNA is converted to the 3’ end and the RNA is protected against enzymes that recognize the 5’end and that break down the RNA.

2-In the event called trimming in the second stage, the parts transferred from DNA to RNA but that are not useful in protein synthesis are trimmed from the RNA by various enzymes.

In the third step called poly-adenylation, a tail consisting of alanine repeats is attached to the 3’ end of the RNA. This tail not does only extend the life of RNA against enzymes that recognize the 3’ end and break down RNA, but also informs that the synthesis has come to an end during protein synthesis.


The following can be stated in the light of all the information above: As a result of a mother’s breastfeeding another child who is not her close relative in addition to her own child, the substances in her milk has direct effects on DNAs and RNAs, which form the basis of the genetic structure of that child.

These changes can be expressed in two stages:

a- The mother’s leukocytes pass to infants with and without blood relation, and the mother’s HLAs (tissue type) on the leukocytes also function in the baby.

b. MicroRNAs belonging to the mother affect the emotional and mother-related genes in a similar way by making the same arrangements on the same genes of the breastfed babies; that is, the babies breastfed by the same mother have the same feeling toward her as the mother.

So, along with our anatomy, our feelings and emotions are created by the effect of the gene regulators in breast milk.

The prohibition of marriage between milk siblings imposed by the religion of Islam, which takes into account this regulatory and equalizing effect of milk on the genetic structure of milk siblings breastfed by the same mother, is suitable and appropriate for wisdom, affairs and scientific approach.

1.I would like to thank my dear brother Prof. Dr. İbrahim Pirim for his valuable contribution

Questions on Islam

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