Intrauterine Growth Restriction (IUGR) is a medical diagnosis given when a fetus fails to achieve its genetically determined growth potential while in the womb. This condition signifies a pathological restriction in growth, making the fetus smaller than expected for its gestational age. IUGR is a serious complication that affects many pregnancies globally, leading to increased health risks for the newborn.
What Exactly Is Intrauterine Growth Restriction?
Intrauterine Growth Restriction indicates that the fetus is not growing at a normal rate due to a pathological process, such as insufficient nutrient supply. This differs from being Small for Gestational Age (SGA), a statistical term used when the estimated fetal weight falls below the 10th percentile for its age. While all fetuses with IUGR are SGA, not all SGA fetuses have IUGR; some are constitutionally small but healthy. The diagnosis of IUGR implies a disease state that has inhibited growth and requires careful medical surveillance.
The condition is categorized into two types based on when the growth insult occurs. Symmetrical IUGR occurs early in pregnancy, resulting in a fetus that is proportionately small in all measurements, including head and abdominal circumference. This type accounts for 20-30% of cases and is often caused by factors intrinsic to the fetus, such as genetic syndromes or early congenital infections. Since the restriction begins during the phase of cell multiplication, the overall number of cells is reduced.
Asymmetrical IUGR accounts for the majority of cases and usually begins later, primarily in the second or third trimester. This type is characterized by “head sparing,” where the fetus prioritizes blood flow and nutrients to the brain to protect neurological development. Consequently, the head circumference may remain near normal, while the abdomen and body length are disproportionately small, reflecting reduced fat growth. The late onset of asymmetrical IUGR is most frequently tied to problems with the placenta’s ability to deliver oxygen and nutrients effectively.
Why Does IUGR Occur?
The underlying reasons for IUGR are diverse and are grouped into three main categories: placental, maternal, and fetal factors. Placental insufficiency is the most frequent cause, occurring when the placenta fails to develop properly or its blood vessels cannot transport sufficient oxygen and nutrients. Conditions such as preeclampsia, which involves new-onset hypertension and organ damage, can severely restrict placental blood flow, causing IUGR.
Maternal health conditions are another contributor to restricted growth. Chronic high blood pressure, pre-existing kidney disease, or severe malnutrition can compromise the environment necessary for fetal development. Exposure to certain substances, including chronic alcohol use, smoking, and recreational drug use, also limits fetal growth potential. Living at a high altitude can also be a factor due to the lower oxygen concentration in the air.
Less commonly, the cause of IUGR lies with the fetus itself. Fetal factors include chromosomal abnormalities, genetic syndromes, or congenital infections like Cytomegalovirus (CMV), rubella, or toxoplasmosis. In these cases, the fetus’s inherent capacity for growth is limited, regardless of the placental blood supply. Additionally, carrying multiple fetuses, such as twins or triplets, increases the demand on the placenta, which can result in growth restriction.
How Doctors Detect and Monitor IUGR
The initial suspicion of IUGR often arises during routine prenatal appointments through the measurement of the uterine fundal height. This measurement, taken from the pubic bone to the top of the uterus, should roughly correlate with the week of gestation after the 20th week. A measurement significantly smaller than expected, often four centimeters or more below the week number, prompts further investigation.
A detailed ultrasound is the primary diagnostic tool used to confirm IUGR and assess its severity. The technician takes multiple measurements, including the head circumference, abdominal circumference, and femur length, to calculate an Estimated Fetal Weight (EFW). IUGR is diagnosed when the EFW falls below the 10th percentile for the gestational age, especially when paired with other signs. Sequential ultrasounds are performed every three to four weeks to track the fetal growth trajectory.
Doppler flow studies are a specialized ultrasound technique that provides information about blood flow dynamics, relevant in cases of suspected placental insufficiency. The Umbilical Artery (UA) Doppler measures resistance in the placenta; increasing resistance is a sign of placental dysfunction. Conversely, the Middle Cerebral Artery (MCA) Doppler assesses blood flow in the fetal brain. A decrease in resistance here suggests the “brain-sparing” effect, meaning the fetus is diverting blood to the brain. The ratio of these two measurements, the Cerebroplacental Ratio (CPR), is an indicator of fetal well-being, with a low CPR associated with adverse outcomes.
Monitoring frequency is adjusted based on the severity of the IUGR and the Doppler findings. For mild cases with normal Doppler results, surveillance may involve twice-weekly testing, including Non-Stress Tests (NST) to monitor the fetal heart rate pattern. More severe cases, particularly those showing abnormal Doppler flow, such as absent or reversed end-diastolic flow in the UA, may necessitate daily monitoring or hospitalization. The goal of this intensive surveillance is to detect signs of fetal compromise early enough to intervene.
Management Strategies and Timing Delivery
Currently, there is no direct treatment that can reverse the underlying cause of IUGR or accelerate fetal growth within the womb. Management focuses on optimizing the intrauterine environment and close surveillance to determine the safest time for delivery. Expectant management includes advising the mother on bed rest to potentially improve uterine blood flow, optimizing nutrition, and controlling any underlying maternal conditions, such as high blood pressure or diabetes.
A major decision in managing IUGR is balancing the risks of the hostile intrauterine environment against the risks of premature birth. If delivery is anticipated before 34 weeks of gestation, a course of antenatal corticosteroids is administered to the mother. These steroids cross the placenta to help mature the fetal lungs, reducing the severity of respiratory distress syndrome, a common complication of prematurity.
The decision to deliver is guided by the results of the Doppler studies and other surveillance tests. For fetuses with IUGR but normal Doppler assessments, delivery can often be delayed until 37 to 39 weeks of gestation. If Doppler studies show increasingly abnormal flow, such as absent or reversed flow in the umbilical artery, delivery must be considered much earlier, often between 32 and 34 weeks, to prevent stillbirth. The ultimate timing is individualized, requiring a careful assessment of the infant’s ability to thrive outside the womb versus the danger of remaining inside.
Short-Term and Long-Term Health Implications
Newborns with IUGR face several immediate health challenges upon birth due to the lack of sufficient energy and nutrient reserves. Short-term complications include a higher risk of low blood sugar (hypoglycemia), difficulty maintaining a stable body temperature (hypothermia) due to low subcutaneous fat, and low oxygen levels (hypoxia) during labor. These infants frequently require admission to the Neonatal Intensive Care Unit (NICU) for specialized care and monitoring.
The long-term implications of IUGR are explained by the theory of “fetal programming” or the Developmental Origins of Health and Disease (DOHaD). This concept suggests that the fetus adapts to the restricted nutrient environment by permanently altering the structure and function of developing organs. These adaptations, while intended for immediate survival, can predispose the individual to chronic diseases later in life.
As adults, individuals who experienced IUGR have an increased risk of developing components of metabolic syndrome. This includes Type 2 Diabetes, hypertension, and cardiovascular disease. The risk is amplified by rapid weight gain in the first years of life, known as “catch-up growth.” Altered epigenetic regulation and changes in vascular function contribute to this lifelong susceptibility.