Fetal Growth Restriction (FGR), also known as intrauterine growth restriction (IUGR), describes a condition where an unborn baby is smaller than expected for its gestational age. This means the fetus is not growing at the typical rate inside the womb, often defined as having an estimated fetal weight or abdominal circumference below the 10th percentile. While some babies are naturally small, FGR indicates the fetus is not reaching its full growth potential due to an underlying reason. Understanding the diverse causes of FGR is important for expectant parents and healthcare providers to identify at-risk pregnancies and plan for appropriate care.
Maternal Health and Lifestyle Factors
Maternal health conditions and lifestyle choices can significantly influence fetal growth by affecting the environment within the womb. Chronic medical conditions in the mother, such as high blood pressure (hypertension), certain types of diabetes, chronic kidney disease, and autoimmune disorders like lupus, can impair blood flow and nutrient delivery to the fetus. Hypertension, for instance, can damage the placenta, reducing its ability to supply necessary oxygen and nutrients.
Maternal infections acquired during pregnancy also pose a risk to fetal growth. Specific infections, including those remembered by the acronym TORCH (Toxoplasmosis, Others like syphilis, Rubella, Cytomegalovirus, Herpes simplex virus), can directly affect fetal development and lead to FGR. Bacterial infections, such as urinary tract infections, are also linked to FGR.
Substance use during pregnancy, including smoking, alcohol consumption, and illicit drug use, significantly contributes to growth restriction. These substances can reduce blood flow to the placenta and compromise the transfer of nutrients and oxygen, creating an unfavorable environment for the growing fetus. Additionally, severe maternal malnutrition or inadequate weight gain throughout pregnancy can deprive the fetus of essential nutrients needed for proper development.
A mother’s age can also play a role, with both very young mothers (adolescents) and those of advanced maternal age potentially having an increased risk of FGR. Older mothers, for example, may have a higher likelihood of pre-existing chronic conditions that impact placental function.
Placental Complications
The placenta serves as the link between mother and fetus, transferring oxygen and nutrients. Complications involving this organ are a common cause of FGR, with placental insufficiency being the most frequent issue. This condition means the placenta does not function optimally, leading to reduced blood flow and nutrient transfer to the developing baby.
Other specific placental problems can also lead to growth restriction. Placental abruption, the premature separation of the placenta from the uterine wall, can compromise the fetus’s blood supply. Placenta previa, where the placenta covers part or all of the cervix, can sometimes lead to reduced placental perfusion, impacting fetal growth.
Abnormal umbilical cord insertion, such as velamentous insertion, also affects nutrient delivery. In this condition, the umbilical cord vessels travel unprotected through the membranes before reaching the placenta, making them vulnerable to compression and rupture. Placental infarcts, areas of dead tissue within the placenta, can further reduce its functional capacity, limiting nutrient and oxygen transfer to the fetus.
In multiple gestations, such as twins or triplets, placental issues can be particularly complex. Twin-to-twin transfusion syndrome (TTTS), occurring in monochorionic twins who share a placenta, involves an unequal sharing of blood. This imbalance often results in one twin receiving too little blood and nutrients, leading to FGR, while the other receives too much.
Fetal Development and Genetic Factors
Fetal Growth Restriction can also stem from factors originating within the fetus itself, including genetic issues or structural problems. Chromosomal abnormalities are a significant contributor to FGR, with conditions like Trisomy 13 (Patau syndrome) or Trisomy 18 (Edwards syndrome) often intrinsically limiting growth potential. These genetic alterations can disrupt normal fetal development and placental function, contributing to restricted growth.
Certain genetic syndromes, even without overt chromosomal abnormalities, can also affect fetal development and growth. These single-gene disorders might lead to FGR by impacting cellular processes or organ formation. Structural birth defects involving major organ malformations, such as severe heart defects, kidney anomalies, or brain abnormalities, can significantly impact overall fetal growth and development.
Congenital infections, passed from the mother to the fetus, can directly interfere with fetal development and cause FGR. Examples include severe cases of cytomegalovirus (CMV), toxoplasmosis, rubella, and Zika virus. These infections can induce inflammation and damage within the placenta and fetal tissues, disrupting normal growth patterns.
When a Specific Cause Isn’t Identified
Despite thorough medical evaluation, a specific cause for Fetal Growth Restriction cannot always be identified. In a significant percentage of cases, FGR is categorized as “idiopathic,” meaning the exact underlying reason remains unknown. Approximately 40% of FGR cases are considered idiopathic, where no clear underlying pathology is found.
For parents, an idiopathic diagnosis does not imply something was overlooked or that they were at fault. This scenario is common, and healthcare providers focus on monitoring fetal well-being. Research continues to advance our understanding of FGR, with ongoing studies dedicated to uncovering the mechanisms behind these unexplained cases.