Spinal muscular atrophy (SMA) is officially recognized as the leading genetic cause of infant death. That sentence tends to land hard — and then the follow-up statistic lands harder: the gene responsible, SMN1, is carried by roughly 1 in 40 to 1 in 60 people, depending on ethnicity, with a panethnic average landing around 1 in 50-54. That's not a rare-disease carrier rate. That's a "you probably know several carriers personally" carrier rate.
Why one copy is fine and two copies isn't
SMA is autosomal recessive, meaning a child only develops the disease if they inherit a non-functional SMN1 gene from both parents. Carriers — people with one working copy and one non-working copy — are completely healthy and asymptomatic. There's no test-free way to know your carrier status; it doesn't show up in a physical, a blood panel, or a family history unless someone in the family has actually had an affected child.
The condition itself progressively destroys the motor neurons that control muscle movement. More than 80% of SMA cases are Type I or Type II — both classified as lethal or severely life-limiting forms, with Type I typically presenting in infancy and carrying a survival rate that drops sharply without early intervention. It's a devastating diagnosis precisely because it's so treatable if caught before symptoms progress — gene therapies approved in recent years can dramatically change outcomes when administered early, which is exactly why newborn screening and pre-pregnancy carrier screening both matter so much.
Both partners are carriers
Each pregnancy carries a 25% chance the child inherits two non-working copies and develops SMA — a 1-in-4 risk per pregnancy, not a one-time roll.
Only one partner is a carrier
No child from that pairing can develop SMA (though children may become carriers themselves) — the risk essentially disappears.
Why ACOG recommends screening everyone — not just "high risk" couples
The American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG) both recommend SMA carrier screening be offered to all women who are pregnant or considering pregnancy — regardless of ethnicity or family history. That's a meaningfully different posture than screening only "at-risk" populations, and it exists because SMA carrier frequency is high and fairly consistent across ethnic groups, unlike some other recessive conditions that cluster heavily in specific populations.
Standard SMN1 deletion testing detects roughly 90-95% of carriers through copy-number analysis. It's a well-established test, not an emerging or experimental one — the challenge has never been the science, it's been the fact that most people planning a pregnancy simply never get asked the question.
Carrier status doesn't announce itself
Whole genome sequencing captures SMN1 copy number alongside hundreds of other carrier-relevant genes in a single test — for you and your partner, before "what if" becomes a delivery-room reality.
Get Your Whole Genome Sequenced → Use code GENOME for 10% off at Dante LabsIf you and your partner are both carriers
- You have options, not a verdict. Preimplantation genetic testing (PGT-M) during IVF can identify unaffected embryos before transfer.
- Prenatal diagnostic testing can determine a fetus's SMA status during an existing pregnancy, giving families time to prepare or pursue early treatment pathways.
- Newborn screening matters even without a known carrier result — many states now screen for SMA at birth specifically because early treatment access changes outcomes so dramatically.
- A genetic counselor should walk you through the numbers — a 25% per-pregnancy risk is very different in practice than a lifetime risk, and it's worth having someone translate that clearly.
A carrier frequency this high, for a condition this serious, with treatments this time-sensitive, is precisely the argument for testing before conception rather than finding out after a diagnosis.