A carrier is a healthy person who has one changed copy of a gene. If both partners carry a change in the same gene, their child could inherit a serious condition. In Thailand, thalassemia is the most important example, and screening before pregnancy gives couples time to consider their choices.
What does being a carrier mean?
We each have two copies of most genes, one from each parent. In recessive conditions, a person needs two changed copies to be affected. Someone with one changed copy is a carrier and usually has no symptoms.
| If both parents are carriers, each pregnancy has | Chance |
|---|---|
| A child who is not a carrier and not affected | 25% (1 in 4) |
| A child who is a healthy carrier | 50% (1 in 2) |
| A child affected by the condition | 25% (1 in 4) |
These chances apply to every pregnancy separately. Having one affected child does not mean the next three will be unaffected.
Thalassemia in Thailand
Thalassemia is a group of inherited conditions that affect haemoglobin, the part of red blood cells that carries oxygen. It mainly affects people of Mediterranean, South Asian, Southeast Asian and Middle Eastern origin.
- Chulabhorn Royal Academy reports that 20 to 30% of Thais carry alpha thalassemia, about 9% carry beta thalassemia and about 13% carry haemoglobin E (HbE)
- Most carriers have no health problems, though some have mild anaemia
- The most severe alpha form, Hb Bart's hydrops fetalis, is usually fatal before or at birth
- Severe beta thalassemia can require regular blood transfusions and treatment for iron overload
Under Thailand's Ministry of Public Health, the Department of Health screens all pregnant women and their husbands at antenatal care to find couples at risk. Testing before pregnancy is even better, because it gives you more options.
Other carrier tests
Thalassemia testing is the priority for most couples in Thailand, but other recessive conditions can also be screened. Which tests make sense depends on your ethnic background and family and medical history.
- Spinal muscular atrophy (SMA): causes muscle wasting and severe weakness
- Cystic fibrosis: affects breathing and digestion; ASRM notes it is more common in people of Northern European descent
- Expanded panels can check dozens of conditions from one blood or saliva sample
No screening test can find every possible condition. A negative result lowers your risk but cannot remove it completely. Ask your doctor or a genetic counsellor what your results do and do not cover.
How screening works
- Blood testFor thalassemia, tests include a complete blood count and haemoglobin typing; DNA testing may be needed for alpha thalassemia.
- Test the partnerIf one partner is a carrier, the other is tested for the same condition. Both results together give the couple's risk.
- Genetic counsellingIf you are an at-risk couple, a counsellor explains what the results mean for your children.
- Choose your pathYou decide which option feels right for your family, with no pressure.
Understanding your results
Cleveland Clinic notes that samples can be blood, saliva or a cheek swab, and that results usually take several days to a few weeks.
If your result is positive, Cleveland Clinic suggests your partner is screened too. What matters is whether you both carry the same condition.
| Result | What it means | Next step |
|---|---|---|
| You are not a carrier | Lower risk for the conditions tested, but not zero | Keep the report for future pregnancies |
| Only one partner is a carrier | For recessive conditions, a child cannot be affected unless both parents are carriers | Test the other partner if not already done |
| Both carry the same condition | Each pregnancy has a 1 in 4 chance of an affected child | See a genetic counsellor to discuss options |
- Ask for genetic counselling after your results
- Discuss all options, including IVF with PGT, donor eggs or sperm, and adoption
- Treat a negative result as a guarantee of a healthy baby
- Assume one partner's result covers both of you
Cleveland Clinic notes that carrier screening cannot detect 100% of carriers for many conditions, and a negative result does not rule out every genetic condition.
Options for at-risk couples
Finding out you are both carriers can be upsetting, but it also gives you choices. Options include natural conception with testing during pregnancy, IVF with preimplantation genetic testing, donor eggs or sperm, or adoption.
With PGT-M, embryos created by IVF are grown to the blastocyst stage. A few cells are taken from the outer layer and tested for the specific condition the parents carry.
PGT cannot guarantee a baby free of all health conditions. Doctors usually recommend confirming the result with CVS or amniocentesis during pregnancy.
What couples should know
Carrier screening is a personal choice, and the results can raise strong feelings. These points can help you prepare.
- The best time to test is before pregnancy; if not, screening should be offered in the first trimester
- Being a carrier is common and is nobody's fault
- Share your results with brothers, sisters and other relatives, who may also be carriers
- Keep a copy of your results for future pregnancies
- Ask questions until you understand your risk and your options
When to see a doctor
- You are planning a pregnancy and have never had thalassemia screening
- You or your partner has a family history of thalassemia or another inherited condition
- One of you is already known to be a carrier
- You have had a previous pregnancy or child affected by a genetic condition
Questions & answers
I feel healthy. Could I still be a carrier?
Yes. Most carriers have no symptoms, which is why a blood test is the only way to know.
If only one of us is a carrier, is our child at risk?
For recessive conditions, the child will not usually be affected, but has a 50% chance of being a carrier.
Do I need IVF to use PGT?
Yes. Preimplantation testing is only available for embryos created through IVF.
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Sources
- ธาลัสซีเมีย ในคนไทย (Thalassemia in Thai people) — Chulabhorn Royal Academy (Chulabhorn Channel)
- สธ. เร่งคัดกรองภาวะโลหิตจาง ค้นหาโรคธาลัสซีเมีย ในหญิงตั้งครรภ์ — Department of Health (กรมอนามัย), Ministry of Public Health Thailand
- Thalassaemia — NHS
- Carrier Screening — Cleveland Clinic
- Preimplantation Genetic Testing (PGT) — Cleveland Clinic
- Genetic Screening for Birth Defects — ASRM ReproductiveFacts.org