Cousin Marriage and Genetic Carrier Screening: A Preconception Guide

All of us may carry a few "silent" changes in our genes that never affect us. This is called being a carrier, and it is no one's fault. Most of the time, being a carrier does not matter. But if the mother and father are carriers of the same condition, the baby can be born with it. A simple blood test before pregnancy lets you find this out in advance and make an informed decision.

How does inheritance work? Conditions like thalassemia and SMA (spinal muscular atrophy) are "recessive". This means a baby has to inherit the changed gene from both the mother and the father to be affected. When both parents are carriers, in every pregnancy:

• There is a 25% chance the baby will be affected
• There is a 50% chance the baby will be healthy but a carrier
• There is a 25% chance the baby will be neither affected nor a carrier

These odds are the same in every pregnancy; one baby's result does not change the next.

Why does being related raise the risk? Because relatives share ancestors, they are more likely to carry the same silent gene. Scientific data show that the risk of a serious genetic condition, about 2-3% for any baby in the general population, rises to about 4-6% when first cousins have a child together. So the risk roughly doubles, but most babies born to cousin couples are healthy. In families where relatives have married each other over many generations, the risk may be higher.

How common is being a carrier?

• Carrier rates for conditions like beta thalassemia and SMA vary by country, population and ancestry.
• These conditions also occur in couples who are not related. That is why screening is meaningful for everyone.

Screening programs:

• Some countries offer carrier screening for hemoglobin disorders (thalassemia, sickle cell) and SMA before marriage or before pregnancy, sometimes free of charge. Often one partner is tested first; if they are a carrier or the result is unclear, the other partner is tested too.
• Couples who are both found to be carriers are referred to genetic counseling.
• If you have not had these tests, or you are not sure about your results, ask your doctor before planning a pregnancy.

International recommendations and expanded screening: The American College of Obstetricians and Gynecologists (ACOG) recommends offering carrier screening for cystic fibrosis, SMA and thalassemia/hemoglobinopathies to everyone considering pregnancy. The American College of Medical Genetics and Genomics (ACMG) recommends offering everyone a broader panel that screens for more than a hundred conditions at once, and considering the most comprehensive panels for related couples. Expanded tests may not be covered by your health system or insurance, so check your coverage. Deciding which test is right for you together with a medical geneticist is best.

The best time: before pregnancy. Screening before pregnancy leaves you the most options and time. If you are already pregnant, it is not too late. In the UK, the NHS recommends this screening ideally before 10 weeks of pregnancy but notes that it can be done at any stage.

What if we are both carriers? First, take a deep breath: this does not mean your baby will definitely be affected. Couples who are both carriers are usually referred to genetic counseling. The options usually explained in counseling are:

• Natural pregnancy and testing during pregnancy: Early in pregnancy, a sample from the placenta (CVS) or the amniotic fluid (amniocentesis) can show the baby's status.
• IVF and preimplantation genetic testing (PGT): Embryos can be tested before they are placed in the uterus.
• No further testing: This is also a valid choice. The NHS stresses that whatever you decide, your healthcare team will support you.

The decision is entirely yours. The counselor's job is to inform, not to direct.

If you are married to a relative: There is no need to feel guilty. If you had carrier screening before marriage, review your results. If there is a known inherited condition in the family, an unexplained baby loss, or problems with intellectual or muscle development, seeing a medical geneticist before pregnancy is especially valuable.

Dear mom-to-be, genetic screening is not there to scare you but to empower you. One tube of blood gives you and your partner the chance to act with knowledge. Whatever the result, there are experts to listen to you and options available to you.