Intrauterine growth restriction (IUGR) happens when a fetus fails to reach its expected size during pregnancy, most often because the placenta isn’t delivering enough oxygen and nutrients. It’s formally defined as an estimated fetal weight or abdominal circumference below the 10th percentile for gestational age on ultrasound. The causes range from problems with the placenta itself to maternal health conditions, infections, fetal genetics, and lifestyle factors, sometimes overlapping in the same pregnancy.
Placental Insufficiency: The Most Common Cause
The placenta is the fetus’s lifeline, and most cases of IUGR trace back to it not working well enough. During a healthy pregnancy, specialized cells called trophoblasts invade the spiral arteries in the uterine wall and remodel them from narrow, high-resistance vessels into wide, low-resistance ones. This happens in two waves: the first around 8 to 10 weeks, and the second around 12 to 14 weeks. When this remodeling is incomplete, blood flow to the placenta stays restricted, and the fetus doesn’t get the oxygen and nutrients it needs to grow on schedule.
Several factors determine how efficiently the placenta transfers substances to the fetus: blood flow on both the maternal and fetal sides, the thickness of the placental tissue, the total surface area available for exchange, and the concentration gradients of nutrients across it. Small molecules like oxygen and carbon dioxide cross based largely on blood flow and placental structure. Larger molecules, including amino acids and glucose, depend on specific transporter proteins embedded in placental cells. In growth-restricted pregnancies, the expression and activity of these transporters is often reduced, meaning less of what the fetus needs actually gets through.
Research in animal models shows just how dramatic these changes can be. Nutrient-deprived placentas can have up to 70% less surface area for exchange due to underdevelopment of the tissue layer where transfer happens, while the barrier between maternal and fetal blood thickens by about 40%. The result is a placenta that is both smaller and less efficient, a combination that progressively limits fetal growth as pregnancy advances.
Maternal Health Conditions
Conditions that affect blood vessels are particularly likely to cause growth restriction because they impair blood flow to the placenta. Chronic high blood pressure and preeclampsia (a pregnancy-specific condition involving high blood pressure and organ damage) are among the most significant. Preeclampsia disrupts the normal remodeling of uterine arteries described above, keeping placental resistance high. It’s a common contributor to IUGR, and pregnancies complicated by preeclampsia are also associated with higher body weight and blood pressure in the child later in life.
Other maternal conditions that raise the risk include diabetes with vascular complications, autoimmune disorders like lupus, kidney disease, and blood clotting disorders such as antiphospholipid syndrome. What these conditions share is that they either damage small blood vessels, promote abnormal clotting in the placenta, or trigger inflammation that interferes with normal placental function.
Smoking and Substance Use
Maternal smoking is one of the most well-documented and preventable causes. An estimated 13 to 19% of full-term babies born at low birth weight can be attributed to prenatal smoking. Nicotine constricts blood vessels, reducing blood flow to the placenta, while carbon monoxide in cigarette smoke binds to hemoglobin and lowers the oxygen available to the fetus. Smokeless tobacco products carry similar risks: infants born to mothers who used them during pregnancy show rates of low birth weight comparable to those seen with cigarette smoking.
Even secondhand smoke exposure during pregnancy is associated with up to a 20% increased risk of having a low birth weight baby. The good news is that quitting makes a measurable difference. Counseling, financial incentives, and cognitive behavioral therapy have all been shown to reduce smoking during pregnancy and lower the risk of low birth weight.
Alcohol, cocaine, and other recreational drugs also contribute to growth restriction through various mechanisms, including direct toxicity to fetal cells and disruption of placental blood flow.
Maternal Nutrition
Poor maternal nutrition during pregnancy directly impairs placental development and fetal growth. When a pregnant person doesn’t consume enough calories or protein, the placenta compensates in ways that backfire. In studies of protein-restricted diets, the transport of amino acids from mother to fetus dropped by 27%, and the activity of key amino acid transport systems in placental cells was significantly reduced. Glucose transport is also affected: severe caloric restriction lowers both maternal blood sugar and the number of glucose transporter molecules in the placenta.
Specific nutrient deficiencies matter too. Growth-restricted placentas show disrupted fat metabolism and reduced levels of two essential fatty acids, arachidonic acid and DHA, both critical for fetal brain and organ development. Protein restriction also reduces the availability of arginine, an amino acid the placenta uses to produce nitric oxide, a molecule that keeps blood vessels dilated and blood flowing freely. Less nitric oxide means more vascular resistance and less nutrient delivery.
Fetal Genetic and Chromosomal Causes
About 20% of growth-restricted fetuses have a chromosomal abnormality, making genetics a substantial contributor. Trisomies 13, 18, and 21 (Down syndrome) are the most commonly associated chromosomal conditions. Other genetic causes include single-gene disorders, confined placental mosaicism (where the placenta has a different chromosomal makeup than the fetus), and conditions involving imprinted genes, which are genes whose activity depends on whether they were inherited from the mother or father. Animal studies have shown that even subtle changes in the expression of imprinted genes involved in placental growth factors can lead to smaller placentas and restricted fetal growth.
Infections During Pregnancy
Intrauterine infections account for roughly 5% of IUGR cases. The classic group is known by the acronym TORCH: toxoplasmosis, rubella, cytomegalovirus (CMV), and herpes simplex virus. These infections can cross the placenta, directly infect fetal tissues, and trigger inflammation that damages the placenta’s ability to function. Growth restriction is a common feature of all of them.
CMV is the most frequent congenital infection in developed countries and can cause damage even when the mother has no symptoms. Toxoplasmosis, typically acquired from undercooked meat or cat feces, and rubella, now rare due to vaccination, can also lead to fetal growth problems alongside structural abnormalities like heart defects and brain calcifications. Malaria is a major cause of IUGR in tropical regions, where placental infection with the parasite impairs nutrient exchange.
Twin and Multiple Pregnancies
Carrying more than one fetus increases the risk of growth restriction simply because two or more babies are competing for the same placental resources. The risk is highest in monochorionic twins, identical twins who share a single placenta. In these pregnancies, selective fetal growth restriction occurs when the placenta’s territory is divided unequally between the two twins. An imaginary line called the “vascular equator” can be drawn along the blood vessel connections between the twins to estimate how much placental territory each one has. When the split is lopsided, one twin thrives while the other falls behind in growth.
This unequal sharing also creates abnormal blood vessel connections between the twins that can further destabilize the balance of blood flow. Dichorionic twins (with separate placentas) can also experience growth restriction, but the mechanism is typically related to one placenta implanting in a less favorable location rather than direct competition for shared tissue.
How These Causes Overlap
In many pregnancies complicated by growth restriction, more than one factor is at play. A mother who smokes and has high blood pressure faces compounding risks to placental blood flow. A fetus with a chromosomal abnormality may also have a poorly developed placenta. Poor nutrition can worsen the effects of preeclampsia. This is one reason growth restriction can be difficult to prevent entirely: even when one risk factor is addressed, others may persist.
There is also no international consensus on exactly when to deliver a growth-restricted baby, particularly in late-onset cases discovered in the third trimester. Guidelines vary by country, and management depends heavily on how severely growth is affected, how far along the pregnancy is, and whether blood flow patterns in the umbilical cord suggest the fetus is in immediate danger. Growth restriction identified earlier in pregnancy tends to be more severe and more likely to have a placental or genetic cause, while late-onset restriction is more often linked to a placenta that functioned adequately at first but couldn’t keep up with the fetus’s accelerating demands in the final weeks.