Intrauterine Growth Restriction (IUGR) is a condition where a fetus fails to achieve its genetically determined growth potential, resulting in a baby that is smaller than expected for its gestational age. A fetus with IUGR is typically defined as having an estimated weight below the 10th percentile for its age in the womb. The underlying causes involve issues with the placenta, the mother’s health, or the fetus itself.
Understanding IUGR Classification
IUGR is broadly categorized into two main types based on how the fetal body is affected. The classification provides clues about the timing and nature of the underlying cause.
The first type is symmetrical, or primary, IUGR, which accounts for about 20% to 30% of all cases. In this classification, all of the fetus’s measurable parts, including the head circumference, abdominal circumference, and femur length, are proportionally reduced in size. This pattern suggests the growth issue began early in the pregnancy, usually in the first trimester. Symmetrical IUGR is frequently caused by intrinsic problems, such as genetic abnormalities or early infections.
The second, more common type is asymmetrical, or secondary, IUGR, making up 70% to 80% of cases. With asymmetrical IUGR, the fetal head circumference is relatively preserved, while the abdominal circumference is significantly smaller. This “head-sparing” effect occurs because the fetus redirects blood flow to the brain and heart at the expense of the liver and abdominal fat. This type of restriction usually results from issues arising later in the pregnancy, often due to a failure in the placenta’s ability to deliver adequate nutrients and oxygen.
Placental and Uterine Circulation Issues
When the placenta functions inefficiently, a condition known as uteroplacental insufficiency occurs, which is the most common mechanical driver of asymmetrical IUGR. This insufficiency often stems from a failure in the normal development of the blood vessels connecting the uterus to the placenta.
A specific failure involves the spiral arteries, small blood vessels within the uterine wall that supply the placenta. Normally, these arteries are remodeled into wide, low-resistance vessels that carry a large volume of blood to the fetus. When this remodeling process is incomplete, the arteries remain narrow and high-resistance, restricting blood flow and causing poor perfusion of the placenta.
Other physical placental conditions can also directly impair blood flow and function. Placenta previa, where the placenta covers the cervix, can lead to bleeding and poor placental integrity. Conversely, placental abruption, the premature separation of the placenta from the uterine wall, can cause sudden and severe restriction of oxygen and nutrients, leading to acute growth failure or fetal distress.
Maternal Health and Environmental Factors
Systemic health conditions in the mother or external environmental exposures can profoundly compromise placental function and fetal growth. Severe chronic hypertension and preeclampsia, a pregnancy-specific condition characterized by new-onset high blood pressure, are significant causes because they constrict the uterine arteries.
Other chronic conditions disrupt the maternal-fetal circulation:
- Severe kidney disease.
- Advanced diabetes.
- Autoimmune disorders.
- Severe anemia or malnutrition in the mother.
Malnutrition limits the availability of oxygen and substrates in the maternal circulation, depriving the fetus of the resources needed for growth.
Environmental factors and substance use pose a direct toxic threat to both the placenta and the fetus. Smoking tobacco is a well-established cause of IUGR, as nicotine and carbon monoxide restrict placental blood flow and reduce oxygen-carrying capacity. The use of alcohol and illicit drugs, such as cocaine or opioids, also acts as a potent vasoconstrictor, severely limiting the nutrient and oxygen supply to the developing baby.
Fetal Structure and Genetic Causes
Some causes of IUGR originate within the fetus itself, leading to the symmetrical growth pattern. These intrinsic issues limit the fetus’s inherent ability to grow, regardless of a healthy placenta or maternal environment. Cell production and organ development are compromised from an early stage, resulting in small fetal measurements.
Chromosomal abnormalities are a common intrinsic cause, with conditions like Down syndrome (Trisomy 21), Trisomy 18, and Trisomy 13 frequently associated with restricted growth. These genetic errors disrupt the developmental pathways required for proper fetal development. Furthermore, congenital structural anomalies, such as severe heart defects or neural tube defects, can divert energy away from general growth or impede the normal functioning of organ systems.
Intrauterine infections caused by viruses and parasites that cross the placenta directly damage fetal cells and organs. This group of infections includes:
- Cytomegalovirus (CMV).
- Rubella.
- Toxoplasmosis.
- Zika virus.
These pathogens infect fetal tissues, leading to cell death, inflammation, and organ damage, which severely restricts growth potential.