A baby can be born without all or part of an arm or leg when the limb is disrupted in the first weeks of pregnancy, while the limbs are first forming. The causes range from a blocked blood supply to a genetic change to, less often, a medicine taken during pregnancy.1PubMed. Limb deficiencies in newborn infants For roughly a third of children doctors never find a reason, and the most common known cause has nothing to do with genes or anything a parent did.
How Common Limb Differences Are and What They Look Like
About 1 in every 1,852 babies in the United States is born with part or all of a limb missing.2Centers for Disease Control and Prevention. Limb Reduction Defects Worldwide estimates are similar, about 5 in every 10,000 live births.3MSD Manual Professional Edition. Congenital Limb Anomalies
Doctors call this a limb reduction defect, meaning part or all of an arm or leg didn’t fully form; you may see this term on your child’s records. One form, phocomelia, leaves the hand or foot attached close to the body because the long bones are short or missing.4PubMed Central. Thalidomide‐induced teratogenesis: History and mechanisms It is very rare, about 6 in every 1 million births.5PubMed Central. Phocomelia: a worldwide descriptive epidemiologic study in a large series of cases from the International Clearinghouse for Birth Defects Surveillance and Research, and overview of the literature Most babies with it have only one affected limb, more often an arm and more often the left side.6PubMed Central. Phocomelia: a worldwide descriptive epidemiologic study in a large series of cases from the International Clearinghouse for Birth Defects Surveillance and Research, and overview of the literature About half have no other birth differences.7PubMed Central. Phocomelia: a worldwide descriptive epidemiologic study in a large series of cases from the International Clearinghouse for Birth Defects Surveillance and Research, and overview of the literature
What parents see varies a lot. Some babies are missing fingers or a hand, others part of a forearm or lower leg. A limb may end cleanly, as if amputated, or be shortened in the middle. Arms are affected more often than legs. Missing all four limbs is extremely rare and usually has a genetic cause.
A single affected limb with no other differences makes a body-wide syndrome less likely, though it doesn’t rule one out. Several affected limbs, or differences in other organs, prompt doctors to look for a genetic or chromosome cause.
How Limbs Form and When Things Go Wrong
Arms and legs begin as small buds around the fourth week of pregnancy and are mostly formed by about week 8. Signals passed between cells tell each bud where to grow, how long to grow, and which side is which.8PubMed Central. Wnt signaling in limb organogenesis One set of signals at the tip keeps the limb growing outward, and another keeps cells moving in the right direction.9PubMed Central. Wnt5a/Jnk and FGF/Mapk pathways regulate the cellular events shaping the vertebrate limb bud If these signals are blocked at the wrong moment, the limb can stop short.
The limb-forming window is early, often before a person knows they’re pregnant. Damage before week 8 tends to cause missing parts, while later problems cause smaller changes. That is why one dose of a harmful medicine at the wrong week can matter. Once the window closes, missing parts can’t regrow later in pregnancy.
Growth proceeds from the shoulder or hip outward to the fingers or toes, so if it stops early, everything beyond that point is missing. A missing middle section with a hand still present suggests a different kind of disruption.
Arms and legs form on slightly different schedules, so an event in a given week can affect an arm and spare a leg, or the reverse. This is one reason doctors can’t read the cause from the appearance alone.
Interrupted Blood Flow and Amniotic Bands
About 35% of limb differences in newborns trace back to interrupted blood flow to the developing limb, making it the leading identified cause.10PubMed. Limb deficiencies in newborn infants A growing limb needs a steady blood supply, and damage to blood vessels in the womb, placenta or baby can cut it off.11PubMed. Structural anomalies resulting from vascular disruption How much damage occurs depends on how big the blockage is and when in pregnancy it happens.12PubMed. Structural anomalies resulting from vascular disruption The part of the limb beyond the blockage may fail to grow or may be lost.
Such a blockage can come from a tiny clot or a small bleed in a vessel, and neither causes symptoms for the mother. Parents often have no memory of anything going wrong, and the first sign is the scan or the birth.
Amniotic bands are one way blood flow gets cut off. In amniotic band syndrome, the inner sac lining tears early, likely before 12 weeks, and leaves loose fibrous strands.13International Journal of Surgery Case Reports. Complete limb amputation due to amniotic band syndrome: Case report from a lower-middle-income country These strands can wrap around fingers, a hand, an arm or a leg and squeeze off its growth and blood flow.14International Journal of Surgery Case Reports. Complete limb amputation due to amniotic band syndrome: Case report from a lower-middle-income country In severe cases, the part of the limb beyond the band is lost before birth.15Journal of Pediatric Orthopaedics / POSNA Study Guide. Amniotic Band Syndrome A baby can have an amputation from bands with no other health problems.16PubMed. Delayed Onset of Osteomyelitis in Fetal Limb Amputation Secondary to Amniotic Band Syndrome: A Case Report
Doctors look for ring-like grooves on fingers, toes, arms or legs, fingers fused at the tips, and clean ends that look like an amputation. Different limbs affected in different ways also suggests bands.
Bands and blood-flow events are random accidents, not caused by diet, exercise or stress. They are generally not inherited, so the chance of recurrence is usually low. A genetic counselor can confirm this after reviewing the pattern.
Genetic Changes and Chromosome Conditions
About 24% of limb differences in newborns come from a single gene change, run in families, or are part of a known syndrome.17PubMed. Limb deficiencies in newborn infants Changes in the genes WNT3 or RSPO2 cause tetra-amelia, a rare condition where all four limbs are missing.18MedlinePlus. Tetra-amelia syndrome Changes in the gene WNT7A can cause a range of limb differences, and a complete loss of the gene causes more severe shortening than a partial loss.19PubMed Central. Mutations in WNT7A cause a range of limb malformations, including Fuhrmann syndrome and Al-Awadi/Raas-Rothschild/Schinzel phocomelia syndrome
About 6% of cases come from extra or missing chromosome material.20PubMed. Limb deficiencies in newborn infants The right amount of chromosome material is needed for normal development, and an imbalance usually affects several body parts.21Journal of Medical Genetics. Heredity and dysmorphic syndromes in congenital limb deficiencies Limb differences occur in Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).22Journal of Medical Genetics. Heredity and dysmorphic syndromes in congenital limb deficiencies Doctors suspect a genetic cause when both sides of the body are affected, or both arms and legs, when there are other differences in the heart, face or organs, or when limb differences run in the family.
Inheritance shapes the chance of it happening again. Some gene changes are passed down, giving a real chance in future pregnancies. Others arise new in the baby and rarely recur. WNT7A conditions need a changed copy from both parents. Chromosome conditions usually occur by chance.
Finding a named gene change or syndrome often ends the search for another cause. It can also tell the family whether other organs, such as the heart or kidneys, should be checked.
Medicines and Other Exposures in Pregnancy
Thalidomide, sold for morning sickness from 1957 to 1962, caused severe birth defects in more than 10,000 children.23PubMed Central. Thalidomide‐induced teratogenesis: History and mechanisms A single 50 mg tablet at the wrong time could cause defects in up to half of pregnancies.24PubMed Central. Thalidomide‐induced teratogenesis: History and mechanisms The drug binds a protein in cells and blocks the signals that keep a limb bud growing.25PubMed Central. Teratogenic effects of thalidomide: molecular mechanisms It also damages growing blood vessels.26PubMed Central. Thalidomide‐induced teratogenesis: History and mechanisms
Other medicines linked to limb differences include warfarin, valproic acid, phenytoin and misoprostol.27PubMed. Prenatal exposure to environmental factors and congenital limb defects Still, medicines and other harmful substances explain only about 4% of limb differences.28PubMed. Limb deficiencies in newborn infants Most common medicines, including ordinary pain relievers and everyday allergy and cold remedies, were not shown to act this way.
A single exposure is also hard to blame. Harm requires the right drug at the right week of limb formation, and most exposures happen outside that window. A doctor can check whether the timing of an exposure fits the type of limb difference.
If you took one of these medicines early in pregnancy, know that many people do before learning they’re pregnant, with no harm. Don’t stop prescribed seizure or blood-thinning medicine on your own; talk to your doctor. Ask whether your medicine is on a known list and whether a detailed scan is needed. If you’re planning a pregnancy on these drugs, ask about safer alternatives.
Pesticides show weak and mixed evidence. Living in a high-pesticide farming county was linked to about twice the usual chance of limb differences in one US study.29PubMed Central. Congenital limb reduction defects in the agricultural setting Another study found no link between parents’ pesticide exposure or farm work and limb differences overall.30PubMed. Potential parental exposure to pesticides and limb reduction defects There is no firm evidence that ordinary pesticide exposure causes limb differences.
Diabetes Before Pregnancy
Mothers with diabetes before pregnancy had about 12 times the usual chance of a baby with a limb difference in one population study.31PubMed. Maternal risk factors for congenital limb deficiencies: A population-based case-control study Pre-existing diabetes also raises the chance of heart, kidney and other birth differences.32PubMed. Risk of birth defects associated with maternal pregestational diabetes Diabetes that develops during pregnancy raises limb-difference risk only slightly, about 14% above normal.33PubMed Central. Risks of specific congenital anomalies in offspring of women with diabetes: A systematic review and meta-analysis of population-based studies including over 80 million births That fits with gestational diabetes usually appearing after the limbs have formed.
Persistently high blood sugar in the early weeks is thought to disturb how the embryo’s cells grow and develop. The risk comes from blood sugar control around conception and early pregnancy, not from diabetes alone.
Because limbs form so early, good blood sugar control before conception matters most. Ask your doctor about a target for HbA1c, the blood test that measures average blood sugar, before trying to conceive. Even 12 times a rare risk still means most babies are unaffected, and an early detailed ultrasound is often offered.
Why Many Cases Have No Known Cause
About 32% of limb differences have no identifiable cause.34PubMed. Limb deficiencies in newborn infants Newer testing is changing that for some families. Whole-genome testing found a likely genetic cause in about 17% of families with limb differences that standard tests had missed.35PubMed Central. Genome sequencing in families with congenital limb malformations In families with inherited-looking limb differences, gene testing explained about 6 in 10.36PubMed Central. Developmental genomics of limb malformations: Allelic series in association with gene dosage effects contribute to the clinical variability
Testing usually needs a blood or saliva sample from the child and often both parents. Results can take weeks to months. A genetic counselor explains what they mean for future pregnancies. A negative result doesn’t rule out a cause, but it often points to a random event.
“No known cause” often points to a chance event in early development, such as a small blood-flow disruption that left no trace. Doctors can usually still describe the likely risk for another pregnancy, and for unexplained single-limb differences it is generally low.
Spotting Limb Differences on Ultrasound
Ultrasound found about 3 in 4 babies with shortened or missing long arm or leg bones.37PubMed Central. The utility of ultrasound for the detection of fetal limb abnormalities – a 20-year single-center experience It found only about 1 in 5 babies with extra or missing fingers or toes.38PubMed Central. The utility of ultrasound for the detection of fetal limb abnormalities – a 20-year single-center experience About 8 in 10 limb differences found before birth were seen in the first trimester.39PubMed. Routine screening for fetal limb abnormalities in the first trimester
Ultrasound relies on seeing small bones, and a baby’s position or the equipment can hide them. Small differences in the fingers and toes are the ones most easily missed. A limb difference can therefore first be recognised at birth even after normal scans, and this is not a sign anyone made a mistake.
After a limb difference is seen, expect a referral for a detailed scan to check other organs. You may be offered genetic testing such as amniocentesis. Meeting a hand surgeon or limb team before birth helps planning, and it’s worth asking whether bands are visible.
How Children With Limb Differences Do
Children with limb differences generally have emotional wellbeing comparable to other children.40PubMed. Participation and quality of life in children and adolescents with congenital limb deficiencies: A narrative review Children with hand differences report less arm function but low anxiety and depression and strong friendships.41PubMed. Functional Impact of Congenital Hand Differences: Early Results From the Congenital Upper Limb Differences (CoULD) Registry Adults born with a leg difference reported better physical function than those who lost a leg to disease.42PubMed Central. Physical and Psychosocial Functions of Adults with Lower Limb Congenital Deficiencies and Amputations in Childhood Children born missing a forearm can learn to control advanced muscle-powered prosthetic hands well.43PubMed. Evaluating the Ability of Congenital Upper Extremity Amputees to Control a Multi-Degree of Freedom Myoelectric Prosthesis
Babies and toddlers usually find their own way of doing tasks, using the other hand, the feet, the chin or the shorter limb. Therapists can help early, but children often work out solutions without being shown. Milestones such as sitting, crawling and walking may come a little differently but typically arrive.
Specialist limb clinics combine surgeons, therapists and prosthetists. Many children manage daily tasks without a prosthesis, and those who use one have it fitted and replaced as they grow. Parent groups help families connect with others who have been through the same thing.