In 2006, a young American named Brooke Greenberg died at the age of 20 having never physically grown past infancy — her body remained that of a two-year-old for her entire life. Her syndrome was never officially named; geneticists couldn't pin down the mutation with certainty. Brooke's story became one of the most striking illustrations of just how varied and unpredictable human genetic variation can be — and how rarely any of us think about it in everyday life, let alone when planning a family.
Carrier screening is a test that shows whether a person carries one copy of a mutation in a recessive gene. The carrier themselves is typically completely healthy: a recessive mutation doesn't express itself when the second copy of the gene is functioning normally. But if both partners happen to carry a mutation in the same gene, the risk of having a child with a serious condition is 25% with each pregnancy.
How carrier status works
Think of it this way: almost every gene in your body comes in two copies — one from each biological parent. If one copy is ‘broken’ (carrying a pathogenic variant), the other usually picks up the slack, and the person lives their whole life without symptoms. But they pass that broken variant on to their children with a 50% probability.
When both parents are carriers of the same mutation, each pregnancy plays out roughly like this: 25% chance the child inherits two working copies (healthy, not a carrier), 50% chance they get one working and one broken copy (healthy, but a carrier like the parents), and 25% chance they inherit two broken copies. That last scenario is where disease develops.
On average, every person carries 2–3 pathogenic recessive variants. We just don’t know about them — and usually never find out.
This isn't rare. According to the American College of Medical Genetics and Genomics (ACMG), expanded carrier screening identifies significant matches in roughly 1 in 30 couples. Some studies put the figure closer to 1 in 20, depending on the panel used and the ethnic backgrounds of participants.
Why ‘just test the donor’ isn’t enough
This is the most important part — especially for same-sex couples and anyone using donor cells.
Imagine a lesbian couple using donor sperm. One partner carries the pregnancy and provides the egg. The sperm bank has tested the donor: no mutations on the standard panel. Everything looks fine, right?
Not necessarily. If the birth mother is a carrier for cystic fibrosis, and the donor is also a carrier — perhaps tested on a narrower panel that missed it — the risk to the child remains. Or consider another scenario: the donor has been fully screened, but the recipient partner carries a pathogenic variant that nobody looked for, because ‘the donor was already tested.’
Genetic risk is always an equation with two variables. Screening one person solves half of it.
For male couples
In surrogacy with a donor egg, there are three people in the genetic equation: both fathers and the egg donor. The biological father passes his genetic material directly to the child. So does the egg donor. The surrogate’s carrier status doesn’t affect the child’s genome (unless she’s also the egg donor), though her genes can influence the intrauterine environment — but that’s a separate conversation.
The practical takeaway: both fathers should be screened. If the biological father and the egg donor both carry the same mutation, that’s critical information to have before the process begins.
For female couples
If one partner provides the egg and the other carries the pregnancy (reciprocal IVF, or RIVF), only one of them passes genetic material to the child — the one whose egg is used. The carrier status of the birth mother doesn’t affect this particular pregnancy. But if one partner’s egg is fertilised with donor sperm, there are two people in the genetic equation: the biological mother and the donor.
The biological mother and the sperm donor both need to be screened — and their results compared — before choosing a donor, not after.
What expanded carrier screening actually tests
Carrier screening used to be targeted: a handful of conditions associated with specific ethnic groups. Ashkenazi Jews were tested for Tay-Sachs. African Americans for sickle cell anaemia. Mediterranean populations for thalassaemia.
Modern expanded carrier screening (ECS) works differently: a single test checks 200 to 500+ genes, regardless of ethnic background. This matters because carrier status doesn’t respect ethnic boundaries as neatly as once assumed — and because many families have mixed heritage.
Among the most clinically significant conditions on expanded panels:
Cystic fibrosis — affects the lungs and digestive system; roughly 1 in 25 Europeans carries a variant
Spinal muscular atrophy (SMA) — progressive muscle weakness; without treatment, one of the leading genetic causes of infant death
Phenylketonuria (PKU) — a disorder of phenylalanine metabolism; fully manageable with diet if caught at birth
Sickle cell anaemia — an abnormality in haemoglobin structure; most prevalent in people of African, Mediterranean, and Middle Eastern descent
Fragile X syndrome (FMR1) — the most common inherited cause of intellectual disability and a leading genetic cause of premature ovarian insufficiency
One important caveat: different labs offer different panels. ‘500 genes’ at one lab and ‘500 genes’ at another aren’t necessarily the same 500. Before testing, it’s worth checking whether the panel covers the specific conditions relevant to your background.
What to do with the results
If one partner tests positive as a carrier — that’s not a catastrophe. It’s information. The next step is testing the other person in the equation (the partner or donor) for the same mutation.
If there’s a match, the couple has several options:
Preimplantation genetic testing (PGT-M): embryos are tested before transfer, and only those without two pathogenic copies are transferred. This is the most reliable way to rule out the risk entirely.
Choosing a different donor: if the donor is a carrier and the intended parent is too, sometimes the simplest solution is selecting a donor who doesn’t carry the same variant.
Prenatal diagnosis: testing the foetus during pregnancy via amniocentesis or chorionic villus sampling gives an answer, but after conception has already happened.
An informed decision to continue: some couples, fully understanding the risks, choose to proceed without additional testing. That’s a legitimate choice — as long as it’s a conscious one.
Knowing you’re a carrier doesn’t change anything about who you are. It changes what decisions you can make.
When to test
Ideally — before anything else starts. Before choosing a donor, before stimulation, before any medical procedures. That’s when you have the most time and the most options.
In practice, many couples do the screening in parallel with their initial workup — as one of the first tests when they begin the process. Many fertility clinics now include ECS as standard for same-sex couples.
If screening reveals a carrier match mid-protocol, the options narrow but don’t disappear. PGT-M can be added at the embryo stage, after fertilisation.
One fact worth sitting with
A 2015 study published in Genetics in Medicine found that expanded carrier screening identified matches in couples 2.5 times more often than traditional targeted screening. The majority of these couples had no family history of the conditions in question.
That’s not a reason to panic. It’s a reason to test.
This article is for educational purposes only and does not constitute medical advice. Before making any decisions related to genetic testing, we recommend speaking with a geneticist or reproductive specialist.