B15. Intrauterine growth restriction (IUGR)

Intrauterine growth restriction (IUGR) or foetal growth restriction (FGR) is a pathological state where the foetus does not achieve its intrauterine growth potential. In other words, the foetus does not grow as much as they would if all factors were optimal. In most cases, IUGR causes a foetus which is small for gestational age (SGA). Small for gestational age is defined as an estimated foetal weight which is less than the 10th percentile for that gestational age and gender.

However, not always. A foetus which was “destined” to be larger than the average (comes from a large family) but for some reason had their intrauterine growth restricted, may have a weight as appropriate for gestational age (AGA). Likewise, some foetuses are small for gestational age but without IUGR, likely because they come from a family of smaller people. These are called constitutionally small for gestational age.

To summarise, IUGR is always pathological, while SGA isn’t necessarily. It’s estimated that 25 – 50% of SGA foetuses are simply constitutionally small. Also, IUGR doesn’t always cause SGA, but in most cases it does.

By diagnosing IUGR, we may reverse or improve any treatable underlying cause, establish more intensive monitoring, and evaluate the need for early induction of labour. The presence of IUGR indicates an increased risk for adverse outcomes, including abnormal neurodevelopment and death.

Etiology

• Foetal
  • TORCH infection
  • Chromosomal abnormality
  • Multiple gestation
• Placental
  • Abnormal placenta or placentation
• Maternal
  • Previous child with IUGR
  • Mother was growth restricted herself
  • Preeclampsia
  • Maternal chronic disease
  • Exposure to environmental factors (smoking, tobacco, pollution, drugs)

Classification

We distinguish symmetrical and asymmetrical IUGR.

Asymmetrical IUGR (or late IUGR) refers to IUGR where the body is disproportionally smaller than the head. This accounts for most cases of IUGR and is usually due to placental or maternal causes, and typically occurs in the later stages of gestation.

Symmetric IUGR (or early IUGR) refers to IUGR where the whole body is smaller, but proportional. This accounts for only 20 – 30% of cases and is usually due to foetal causes. It typically occurs in the early stages of gestation.

Diagnosis and evaluation

During routine prenatal care visits, the foetus’ weight and growth over time is estimated with the help of the symphysis-fundal height and ultrasound. Any foetus which is estimated to be small for gestational age or shows diminishing growth velocity should be evaluated for IUGR.

Thorough history and ultrasound examination with umbilical artery Doppler must be performed. NST and CST may also be performed.

The following findings suggest a constitutionally small foetus, rather than IUGR:

• Modest smallness (weight lower than 10th percentile, but not lower than 5th percentile)
• Normal anatomy
• Normal growth velocity
• Normal Doppler examination of the umbilical and middle cerebral artery
• Normal amniotic fluid volume
• Appropriate foetal size in relation to maternal size characteristics

Abnormal arterial flow in the umbilical artery (determined by Doppler) suggests that placental insufficiency is present. Abnormal anatomy, early IUGR, or polyhydramnios suggests chromosomal abnormality.

Treatment

Any underlying reversible causes (preeclampsia, smoking, infection, etc.) should be treated if possible. Apart from that, the management involves choosing between continued close monitoring or induced delivery. The choice depends on the degree of severity, the gestational age, and the CTG and ultrasound findings. In general, delivery is indicated if the risk for in utero demise is larger than the risk for perinatal demise.

Prevention

Interestingly, a 2021 randomised clinical trial found that pregnant women at high risk for SGA who were treated with a Mediterranean diet or mindfulness-based stress reduction significantly reduced the percentage of SGA newborns.

Prognosis

The presence of IUGR indicates an increased risk for adverse outcomes, including abnormal neurodevelopment and death. There may also be longer term outcomes, including obesity, diabetes, cardiovascular disease, etc. Poor prognostic factors include:

• Early IUGR
• More severe growth restriction (severe smallness)
• Abnormal umbilical artery blood flow on Doppler