Abstract
A woman lies on an exam table, holding her partner’s hand tightly with anticipation, as a technician glides an ultrasound probe across her abdomen. On the screen, shifting staticky shadows resolve into a skull, a liver, and the flicker of a beating heart. For many families, this moment brings joy and relief. For others, it’s paralyzing, as doctors detect signs that something is wrong.
A single nucleotide change can cause neurodevelopmental delays and dimorphism, failing livers, and arrhythmia-ridden hearts. For decades, medicine could only identify these conditions, usually after birth. Prenatal screening has made it easier to detect progressive diseases like Duchenne muscular dystrophy, which degenerates and damages muscles before symptoms typically appear in childhood. But treating before birth could preserve tissue prior to the onset of irreversible deterioration.
Once unthinkable, genetic diseases can now be treated before birth. Fetal genetic intervention—including early screening, in utero gene therapy, stem cell transplantation, and even embryo editing—aims not just to diagnose disease but to correct it at its earliest stages. It is a rapidly advancing frontier, defined by technological promise and profound ethical questions.
It starts with detection
Jennifer Hoskovec, vice president of medical affairs at BillionToOne, has spent more than 20 years in prenatal genetics, an era dominated by risk assessment rather than intervention.
Jennifer Hoskovec
Vice President
BillionToOne
Historically, prenatal genetic screening has fallen into two main categories. Aneuploidy testing determines the risk of Down syndrome and other trisomies, sex chromosome abnormalities, and specific microdeletions. Screening is essential for these de novo mutations, which have no U.S. Food and Drug Administration (FDA)-approved genetic interventions. High-risk Down syndrome patients may receive a fetal echocardiogram, closer ultrasound monitoring, or tertiary care delivery with neonatal support. The standard practice is to screen, monitor, and manage.
The second category involves inherited recessive conditions like cystic fibrosis (CF), spinal muscular atrophy (SMA), and phenylketonuria. If both parents are carriers for the same genetic mutation, then their child has a 25% chance of being affected. Testing typically requires samples from both parents. If both are carriers, chorionic villus sampling (CVS) and amniocentesis can detect fetal abnormalities in the first and second trimesters, respectively. However, getting each partner to follow up is a major hindrance. “When people go through a screening process and are found to be carriers, less than 50% of their partners complete the testing,” Hoskovec told Inside Precision Medicine. “Half of U.S. carriers of these genetic conditions, whether common or rare, don’t know what it means for their pregnancy. That limits their ability to get diagnostic testing because we do not have all the pieces of the puzzle.”
Hoskovec’s team developed a workaround: a single-gene noninvasive prenatal test that analyzes fetal cell-free DNA (cfDNA) circulating in maternal blood. Around nine weeks into pregnancy, fragments of fetal DNA shed from the placenta can be sequenced and quantified. If a mother is a carrier for a condition like CF or sickle cell disease, the test looks for a second variant that is not present in her DNA and forms evidence of paternal contribution.
“For example, if a mother has [the] sickle cell trait, we first sequence the full beta-globin gene in the cfDNA, which contains a mixture of maternal and fetal DNA,” Hoskovec said. “We look for a second variant not present in the mother that would indicate paternal contribution.”
Despite not replacing CVS or amniocentesis, Hoskovec said the result is highly sensitive, identifying 95% of affected pregnancies in the conditions it covers. Crucially, it does not require partner testing. “This is a stepping stone,” Hoskovec explained. “This earlier detection will likely accelerate the field by increasing the number of eligible patients for clinical studies and registries, improving equitable access across ethnic groups, and advancing precision medicine in prenatal care.”
Avoiding germline editing
As screening opens the door, fetal surgeons and gene therapy researchers are taking their first steps through it. A pediatric surgeon at the Yale School of Medicine, David H. Stitelman, MD, believes prenatal treatment has benefits. The fetus is small, so it can receive higher doses based on weight. As its immune system is still developing and more tolerant, stem cells are growing quickly and organs are still being formed, so problems can be fixed before they become permanent. Because the placenta exchanges oxygen, lung conditions like congenital diaphragmatic hernia can be treated during fetal life. But once a newborn takes a first breath, defective lungs can spell immediate crisis.
David H. Stitelman, MD
Associate Professor
Yale-New Haven Children’s Hospital and Yale School of Medicine
Fetal therapy is not new. Specialized centers have performed open fetal surgery for spina bifida and diaphragmatic hernia lung growth, and blood transfusions for fetal anemia dating back to the 1960s. What is new is the molecular toolkit. Stitelman’s lab is investigating gene editing methods that use the cell’s repair machinery to fix one- to three-base-pair DNA errors. Another team, led by pediatric and fetal surgeon Tippi MacKenzie, MD, at the University of California, San Francisco, is using viruses to replace genes for lysosomal storage diseases and fetal stem cells for alpha thalassemia.
Some diseases require only modest correction. In hemophilia, one percent normal clotting factor expression improves outcomes greatly. Increasing the expression of functional CFTR protein to 15% of wild-type levels may cure CF or at least make it manageable. Even a small number of liver cells corrected in hereditary tyrosinemia can boost growth and repopulate the organ. However, some situations, such as congenital cancer syndromes, may require nearly 100% correction. At present, Stitelman’s team achieves single-digit percentage editing in models of CF and beta thalassemia. “We’re in the optimization phase,” Stitelman told Inside Precision Medicine. “We are testing different nanoparticles and generations of editing strategies to incrementally reach therapeutic levels.”
Stitelman draws a clear ethical boundary: this is somatic editing, not germline editing. The aim is to treat the fetus as a patient, not to create heritable genetic changes. Instead of editing embryos in vitro, systemic therapeutic agents are delivered to avoid reproductive cell damage.
Unintended germline modification remains a concern. Editing a target gene could inadvertently disrupt developmental genes and affect future generations. But, Stitelman argues, medicine always carries risk. “In 1950, children with leukemia all died,” said Stitelman. “Today, some forms have a 98% long-term survival rate with chemotherapy. We know chemotherapy can cause germline mutations, yet we accept that risk because it saves lives. With gene editing, the issue is not zero risk but understanding and quantifying the risk. Ideally, there would be no measurable off-target effects. In the places we have examined, we have not seen off-target effects.”
One pregnancy, two patients
In a landmark trial in 2011 known as the Management of Myelomeningocele Study, investigators found that fetal surgery for severe spina bifida (myelomeningocele) achieved better results than postnatal repair. Surgically closing the spinal defect in utero improved motor function and reduced the need for shunting to relieve hydrocephalus. The benefit was so clear that the trial was stopped early and influenced how doctors treat structural birth defects.
At the University of California, Davis, biomedical engineer Aijun Wang, PhD, is working closely with fetal surgery pioneer Diana L. Farmer, MD, to evolve fetal intervention from heroic surgery to cellular and molecular therapy. Wang and Farmer launched the Cellular Therapy for In Utero Repair of Myelomeningocele (CuRe) trial, combining fetal surgery with stem cell transplantation. The goal is to not only close the spinal defect but also restore neural tissue and improve long-term function.
Aijun Wang, PhD
Professor
University of California, Davis (UC Davis)
The lens that Wang has used to focus his research is fetal and maternal safety. “The fetus is the patient, but treatment inevitably carries some risk to the mother,” Wang told Inside Precision Medicine. “Open fetal surgery, in particular, poses significant maternal risk. Genetic treatments introduce additional uncertainties because the long-term effects of DNA modification are not fully understood. Safety must remain the highest priority.”
Genetic medicine delivery is a critical challenge for all life stages, but the stakes are particularly high for a developing fetus. In fetal development, targeting stem cell populations is especially important because these cells are highly active, proliferating, and migrating. If edited successfully at the right developmental window, their progeny will carry the correction. The problem would be if the edit was not just unsuccessful but detrimental.
Wang’s lab focuses on delivery systems, particularly lipid nanoparticles carrying mRNA-encoding gene-editing enzymes. For genetic manipulation and high-throughput screening, Wang’s lab utilizes mouse models. Fetal sheep are used for scaling and dosing, while human organoids are used for human-specific editing and functional outcomes.
“In our clinical work, we have engaged with the FDA and conducted extensive preclinical studies,” said Wang. “Using multiple complementary models is essential. Combining small animal models, large animal translational models, and human organoid systems provides a comprehensive framework for product development, from early screening to human-focused therapeutic design.”
Although the field is highly exciting and progressing rapidly, Wang warns against premature application, which could be dangerous. Safety, developmental biology, ethical considerations, and multidisciplinary collaboration are all essential. “Despite the excitement in the field, we must proceed cautiously,” said Wang. “There is strong potential for correcting specific mutations, especially point mutations, using precise gene editing approaches such as base editing. However, safety evaluation must precede rapid clinical application.”
Norbert Gleicher, MD
Founder & Medical Director
Center for Human Reproduction
Effective progress requires a village of physicians, surgeons, researchers, engineers, and ethicists working together. Scientific progress requires caution, responsibility, and thorough evaluation before clinical use.
The earlier, the better
If fetal intervention treats a diagnosed fetus, embryo editing operates even earlier—at the blastocyst stage in in vitro fertilization (IVF). Norbert Gleicher, MD, a fertility specialist known for treating some of the oldest and most difficult IVF patients in the United States, approaches genetic technologies with caution. Due to biological mosaicism, sampling limitations, and his belief that many abnormal embryos self-correct or develop normally, Gleicher opposes preimplantation genetic testing for aneuploidy.
But when it comes to single-gene diseases, he sees a different calculus. Couples with recessive mutations may have one-in-four embryos affected, and in dominant or X-linked diseases, half may carry the mutation. For patients who produce few embryos—especially older women—discarding affected embryos can mean losing precious chances at pregnancy. “If you can cure an embryo rather than discard it,” Gleicher told Inside Precision Medicine, “that makes a lot of sense.”
For single-gene diseases, Gleicher believes genetic editing with CRISPR or other platforms is the most straightforward intervention. He points to the 2025 work at the Children’s Hospital of Philadelphia on Baby KJ as a recent milestone. Even partial correction, which Gleicher believes is likely the case with Baby KJ—though no liver biopsies have been extracted—can transform prognosis. Gleicher said, “Correcting some cells was enough to clinically cure the baby, at least for the time being, from symptoms of a disease that historically kills affected children within a few years. However, we do not know whether the treated baby, who likely still has many affected cells, might become symptomatic again later in life.”
To Gleicher, success in a newborn is all the more reason to apply genetic intervention to fetal stages. “If this can be successful in a full human being, imagine how much easier it would be at the blastocyst stage, or even earlier at the cleavage stage, when the embryo consists of only six to eight cells,” said Gleicher. “If [CRISPR] is applied at that point, correcting those six to eight cells would mean that all their daughter cells would also be corrected. The result would be a normal baby at birth. That is the much stronger argument in this case.”
Just because something is possible, it doesn’t necessarily mean it should be done, and Gleicher establishes a clear ethical boundary. Editing to prevent a devastating single-gene disease is one thing. Editing for traits—eye color, intelligence, polygenic risk scores—is another. Polygenic predictions explain only a fraction of trait variance, and embryo implantation itself is uncertain. To him, offering polygenic selection in IVF is not only scientifically dubious but also ethically troubling. “It is surprising that professionals, particularly in genetics, would suggest such an approach,” said Gleicher. “It is worse than snake oil, because while snake oil may occasionally work by accident, this carries a real risk of causing serious harm.”
A pretty penny
What ultimately restricts fetal genetic intervention is timing. Early screening increases experimental trial eligibility, and early treatment may preserve organ development before irreversible damage. In conditions like CF and SMA, where postnatal gene therapies are expensive and delivered after injury, fetal intervention could change outcomes. Frontline screening can identify high-risk pregnancies at 11 weeks without family history or ethnicity, expanding trial access.
Yet, fetal genetic interventions require specialized teams, advanced delivery systems, counseling, and long-term follow-up. Without careful planning and reimbursement policies, only a few top-tier centers could progress, widening the gap. Ethical scrutiny remains inseparable from progress. Innovation must balance maternal risk, fetal benefit, and future consequences with safety, appropriate use, and clear limits. As prenatal care shifts from prediction to prevention, restraint and evidence will determine its future.