Consanguinity is the medical word for being related by blood. It comes up most often when two relatives, usually cousins, have children together. About 1 in 10 people worldwide are in, or descended from, such a marriage, so this is an everyday family situation in much of the world.1Annual Review of Anthropology. Consanguineous Marriage and Human Evolution The added health risk for their children is real but smaller than many people assume, and most children of cousins are born healthy. What matters most is which genes a particular family carries, which is why testing can tell a couple far more than averages can.
Degrees of Blood Relation and What Counts
Doctors and researchers usually count a couple as consanguineous when they are second cousins or closer.2PubMed Central. Genomic Runs of Homozygosity Record Population History and Consanguinity First cousins, the children of siblings, are the most common consanguineous couples worldwide. They share about one-eighth of their DNA through their shared grandparents. Second cousins share about one thirty-second, so their children’s added risk is much smaller. Uncle-niece unions share about a quarter, and relationships closer than that are banned almost everywhere.
To work out your own degree, trace back to the nearest ancestor you and your partner share. Shared grandparents make you first cousins, and shared great-grandparents make you second cousins. “Once removed” means one generation apart, such as your parent’s cousin. Half-cousins, who share only one grandparent, share about half as much DNA as full first cousins.
Some families have a more tangled tree. Where cousin marriage has repeated over generations, a couple may be related through several lines and can share noticeably more DNA than ordinary first cousins. Telling a doctor about every family link gives a truer picture of the risk.
The percentages above are averages. DNA is shuffled at random each generation, so two particular first cousins can share a little more or less than one-eighth.
Even unrelated people share some DNA. Everyone inherits some identical stretches from both parents, because all families trace back to shared ancestors far in the past.3PubMed. Runs of homozygosity: windows into population history and trait architecture Children of close relatives inherit many more of these stretches, and much longer ones.4PubMed Central. Runs of Homozygosity Implicate Autozygosity as a Schizophrenia Risk Factor
The word itself describes how two people are related and says nothing about whether either partner has a genetic condition. A couple can be consanguineous and share no faulty genes at all, so the term marks a higher chance, not a diagnosis.
Why Being Related Raises Genetic Risk
Many genetic conditions are recessive: they appear only when a child gets the same faulty gene from both parents. Someone with one faulty copy is a carrier and is usually perfectly healthy. Most people carry a few such hidden genes without knowing it. When two carriers of the same gene have a child, each pregnancy has a 1 in 4 chance of the condition.
That 1 in 4 applies separately to every pregnancy, so having one affected child does not “use up” the risk or make the next child safe. Most children of two carriers are not affected.
Relatives can inherit the same faulty gene from a shared ancestor, so they are more likely to be carriers of the same one. A faulty gene in one grandparent can travel down two separate family lines, giving two cousins a real chance of holding the same copy. Unrelated people from different backgrounds usually carry different hidden genes, so their copies rarely match. The main added risk therefore comes from a recent shared ancestor, though some ethnic groups do share particular carrier genes more often.
The result is that being related raises the share of a child’s genes that come in identical matching pairs.5Atlas of Genetics and Cytogenetics in Oncology and Haematology. Consanguinity Harmful gene changes turn up mostly in those long matching stretches inherited from recent shared ancestors.6PubMed Central. Inbreeding and deleterious variation in South Asian populations
Being related does not create new faulty genes. If neither partner carries a particular faulty gene, relatedness adds no risk for that condition. A family’s history of a recessive condition matters far more than the relationship by itself.
It also explains why a condition can appear without warning. Both parents can be healthy carriers with no known affected relatives, because the gene can pass silently for many generations.
How Much Higher the Risk Is for Children of Cousins
Children of first cousins have about 2 to 2.5 times the general risk of birth defects, mostly from recessive conditions.7PubMed Central. Consanguineous Marriage and Its Association With Genetic Disorders in Saudi Arabia: A Review The extra risk works out to about 1.7 to 2.8 more children with a defect per 100 births.8PubMed Central. Consanguineous Marriage and Its Association With Genetic Disorders in Saudi Arabia: A Review In the general population, about 2 to 3 in 100 babies are born with a significant birth defect. First-cousin couples still have roughly a 95 in 100 chance of a child without a major defect. “Double” sounds alarming, but the absolute difference is a few children out of 100.
An average also spreads across many families: most have no added risk, and a few have much more. A couple who carry a matching faulty gene face 1 in 4 for that condition, far above the average.
A family’s own history changes the picture. For first cousins whose last child had no birth defect, the next child’s risk was about 36 in 1,000, under 4 in 100.9PubMed. Consanguinity and recurrence risk of birth defects: a population-based study If an earlier child had a birth defect, the next child’s risk was nearly double, about 68 in 1,000.10PubMed. Consanguinity and recurrence risk of birth defects: a population-based study That jump suggests a shared genetic cause may be at work in that family, which is why a past birth defect is a good reason to ask for genetic evaluation before another pregnancy.
Figures differ between countries and studies. Families where cousin marriage has repeated for generations tend to show higher rates. Poverty, young maternal age, and short gaps between births also raise risk and get tangled up with relatedness.
For infant death, children of first cousins have about 1.1 more deaths per 100 births, across many populations.11PubMed. The impact of consanguinity on neonatal and infant health Some of that gap may come from poverty and access to care rather than genes.12PubMed. The impact of consanguinity on neonatal and infant health Good prenatal care and newborn screening can reduce the part that comes from treatable conditions.
Pregnancy loss shows a similar pattern. In one Indian study, it was 11 in 100 among related couples versus about 4 in 100 among unrelated couples.13PubMed Central. Consanguinity, prematurity, birth weight and pregnancy loss: a prospective cohort study at four primary health center areas of Karnataka, India In that study, related couples also had about one and a half times the odds of stillbirth, mainly for babies born early.14PubMed. Consanguineous marriage, prepregnancy maternal characteristics and stillbirth risk: a population-based case-control study
Conditions Linked to Blood Relation
Inherited metabolic disorders, where the body can’t break down certain substances, are especially linked to related parents.15PubMed Central. The impact of consanguinity on the frequency of inborn errors of metabolism In Denmark these disorders were about 25 times more common among children from communities where cousin marriage is common.16PubMed Central. The impact of consanguinity on the frequency of inborn errors of metabolism This compares whole communities, so it does not mean any given cousin couple has a 25-fold risk.
In everyday terms, a child with a metabolic disorder lacks a working enzyme and cannot process a certain food component or chemical, so it builds up to harmful levels. Babies are often healthy at birth and become unwell after days or weeks of feeding. That is why screening uses a small blood sample taken soon after birth, and why newborn screening for treatable metabolic conditions matters for these families.17PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors Inherited hearing loss is another condition doctors check for in babies of related parents.18PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors
Many of these conditions can be treated if found early. Several metabolic disorders are managed with special diets or medicines. Early hearing detection lets a child get hearing aids or implants before speech is affected. Finding a condition in one child can also lead to testing for siblings and relatives.
Poor feeding, slow growth, missed milestones, or not reacting to sounds are worth mentioning to a doctor. Say that the parents are related, so the doctor thinks about recessive causes early.
Other effects are smaller. Children of first cousins average roughly 1 to 3 cm shorter than children of unrelated parents.19PubMed Central. Evidence of Inbreeding Depression on Human Height In one study in India, children from closely related families scored lower on IQ tests, more so the closer the relation.20PubMed Central. Estimating the Inbreeding Depression on Cognitive Behavior: A Population Based Study of Child Cohort In one tribal community in India, children of related parents had about two and a half times the odds of heart disease, diabetes, and high blood pressure.21PubMed Central. Impact of consanguinity on cardio-metabolic health and other diseases: findings from an Afro-Indian tribal community These come from single communities and don’t set the typical risk for every family.
Genetic Counseling and Testing for Related Couples
There are no evidence-based rules for related couples, so advice rests on expert consensus.22PubMed. Genetic counseling and screening of consanguineous couples and their offspring practice resource: Focused Revision Experts recommend a thorough family medical history first, with follow-up of anything significant.23PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors Related couples should be offered the same screening as any couple from their ethnic background.24PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors During pregnancy, they should be offered blood screening tests and a detailed ultrasound.25PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors After birth, babies should be screened for hearing and treatable metabolic disorders.26PubMed. Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors
The detailed ultrasound, usually done in the middle of pregnancy, looks at the baby’s organs and structure. It can find many structural birth defects, such as heart or spine problems. It cannot detect most recessive conditions, such as metabolic disorders, where the baby’s body looks normal. A normal scan is reassuring but not a guarantee.
Broader tests are now available. Carrier screening checks whether partners carry the same faulty genes, and the expanded version can check almost all genes for recessive conditions at once.27PubMed Central. Diagnostic exome-based preconception carrier testing in consanguineous couples: results from the first 100 couples in clinical practice That suits related couples, who are especially likely to share very rare carrier genes that standard panels miss.28PubMed Central. Diagnostic exome-based preconception carrier testing in consanguineous couples: results from the first 100 couples in clinical practice Learning they share one lets a couple make an informed choice about pregnancy.29PubMed Central. Diagnostic exome-based preconception carrier testing in consanguineous couples: results from the first 100 couples in clinical practice IVF with embryo testing lets couples who share a carrier gene select unaffected embryos.30PubMed Central. Features of chromosomal abnormalities in relation to consanguinity: analysis of 10,556 blastocysts from IVF/ICSI cycles with PGT-A from consanguineous and non-consanguineous couples Clinics need to know the partners are related, since it can affect embryo test accuracy.31Fertility and Sterility. Routine detection of embryonic consanguinity using single-nucleotide polymorphism-array-based preimplantation genetic testing
A negative carrier result greatly lowers the risk but cannot remove it. If both partners carry the same gene, options include natural pregnancy with prenatal testing, IVF with embryo testing, donor eggs or sperm, or adoption. A counselor explains these without telling you what to choose.
The best time for a first genetics appointment is before pregnancy, when every option is still open, including testing and IVF. Seeing a genetic counselor during pregnancy is still worthwhile, since prenatal tests can be arranged and the baby’s care planned. Your family doctor or obstetrician can usually refer you.
To prepare, write down illnesses, early deaths, stillbirths, and disabilities across both sides of the family. Note exactly how you and your partner are related, including any repeat links. Bring test results from relatives who have had genetic diagnoses, and ask whether expanded carrier screening is covered where you live.
How Common Cousin Marriage Is and Why Families Choose It
Rates vary hugely between populations, which shapes how common rare recessive diseases are.32Journal of Rare Diseases. The impact of consanguinity on human health and disease with an emphasis on rare diseases About 1 in 5 people from South, Central, and West Asian populations show genetic signs of closely related parents.33PubMed Central. Genomic Runs of Homozygosity Record Population History and Consanguinity Where cousin marriage is a long-standing custom, rates can be high across whole communities, while in many Western countries it is uncommon. Migrants may keep the custom after moving, so families in countries where it is rare can still be affected. A doctor there may not think to ask, so mentioning it yourself helps.
Cousin marriage has been declining overall, especially in wealthier countries.34Annual Review of Anthropology. Consanguineous Marriage and Human Evolution Rates have fallen with more education, urban living, smaller families, and later marriage.
It has long helped families keep property, trust, and lineage together.35Nature & Anthropology. Consanguineous Marriage in Global Perspective: Anthropological Roots, Genetic Risks, Contemporary Relevance, and the Way Forward In Pakistan, though, people mostly name social reasons rather than money or religion.36PubMed. Community perceptions of reasons for preference for consanguineous marriages in Pakistan Families often prefer a partner they already know, whose family, health, and values are familiar. Marrying within the family can keep a bride near her relatives and simplify arrangements about property and dowry. For many couples it is simply a personal choice, or a love match that happens to be between relatives.
Related couples can face stigma, which can make honest talks with doctors harder.37Scholar Commons. Exploring Genetic Counselors’ Experiences, Language, and Discussion of Consanguinity in Clinical Practice: A Multinational Perspective Women found to be carriers may be seen as less suitable for marriage in their community.38PubMed Central. Reproductive choices: a qualitative study of Dutch Moroccan and Turkish consanguineously married women’s perspectives on preconception carrier screening Couples can ask that results be kept private within the clinic, and a counselor who knows the community’s customs can make the conversation easier.
Laws differ by place. First-cousin marriage is legal in most countries, and some US states ban or restrict it. A 2025 bill in the UK proposed banning new first-cousin marriages without voiding existing ones.39PubMed Central. Consanguineous Marriage: Law and Public Health If you plan to marry, check local law, especially when moving countries.