Placenta | Fetal / Intrauterine growth restriction (FGR / IUGR)
Fetal growth restriction (FGR), also known as intrauterine growth restriction (IUGR), occurs when a fetus fails to attain its pre-determined growth potential, as it does not grow at the normal, expected rate.
- Only a proportion of fetuses that are small for their gestational age will be found to have FGR.
- While FGR may be apparent visually (poor maternal weight gain or small abdomen), it can also be diagnosed using ultrasound and Doppler studies (studies of blood flow).
- Some growth restricted fetuses may not tolerate labour well and may need to be delivered by Caesarean section.
- Most such babies do very well after delivery. Their growth will usually ‘catch-up’ to babies born at higher birth weights during their first year of life.
FGR is usually a disorder of placental insufficiency, where the placenta is not providing sufficient nutrients to the fetus to sustain its normal growth.
- It can also be caused by other factors such as abnormal chromosomes or genes, congenital infections, substance abuse (including smoking), maternal conditions, such as high blood pressure, kidney or immune system disorders and other rare causes.
Placental insufficiency can be inferred from a review of the ultrasounds, together with assessment of the fetal development. After birth, review of the placental pathology is extremely important and is key to confirming the diagnosis.
Fetuses with FGR caused by placental insufficiency often develop asymmetrically, that is, the ratio of the fetal head circumference (HC) to abdominal circumference (AC) is increased (the head grows at the normal rate while the abdomen lags behind because the body organs, like the liver, grow slower). Asymmetrical growth also includes a short femur length (FL). This is because fetuses with FGR will usually prioritize which parts of their body most need to be supplied with blood.
Small fetuses in healthy mothers with a normal HC/AC ratio and normal FL may simply be smaller than normal due to their mother’s size and ethnicity.
Severe placental insufficiency has approximately a 10% recurrence risk in otherwise healthy women with the same partner.
FGR can also be caused by placental insufficiency or these factors:
- Abnormal chromosomes
- Abnormalities in a cluster of genes or a single gene
- Silencing of normal genes (called “epigenetic gene silencing”)
- Syndromes with an unknown but presumed genetic basis
- Maternal infections passed to the fetus (congenital infections)
- Substance abuse (including smoking)
- Pregnancy at high altitude
- Starvation (very rare in developed countries)
- Maternal conditions, such as anaemia (low blood count), high blood pressure (hypertension), kidney, cardiac or immune system disorders
- Other rare causes
The signs and symptoms of FGR include poor maternal weight gain, development of hypertension (up to 50% of mothers with FGR pregnancies develop hypertension due to associated placental insufficiency), reduced fetal activity and a small-for-dates pregnancy.
Diagnosis is made using:
Ultrasound: Ultrasound is used to monitor fetal growth and body proportions (e.g. abdominal vs head circumference).
Estimated fetal weight (EFW): EFW is estimated by measuring the fetal head, abdominal circumference (AC), and femur (thigh bone) length (FL) on ultrasound. The average size of male and female Canadian babies (including all ethnic backgrounds and parental height) is used as a reference to determine if the developing fetus is too small (see Canadian growth centiles). If the fetus is below the 10th percentile, this is termed small-for-gestational age (SGA) and a proportion of these fetuses will be found to have true FGR.
Monitoring of fetal body proportions: Fetuses with FGR caused by placental insufficiency often develop asymmetrically, that is, the ratio of the fetal head circumference (HC) to abdominal circumference (AC) is elevated (the head remains a normal size while the abdominal girth lags behind).
Doppler studies: Abnormal blood flow to the fetus can be a sign of FGR. Doppler studies (studies of blood flow) may be used throughout pregnancy to determine whether blood flow to the fetus is normal.
Serial ultrasounds of fetal growth, well-being, and amniotic fluid volume (AFV), in addition to Doppler tests of umbilical, brain, heart and uterine vessel blood flow are essential. The frequency of these assessments will be determined based on the severity of FGR.
Antenatal steroids (glucocorticoids) may be administered to a growth restricted fetus, to prepare its lungs for early delivery, with close daily monitoring, sometimes as an inpatient.
In some cases of severe early-onset FGR, the Caesarean section may require a vertical uterine incision (called a “classical” Caesarean section), because the fetus is too small for the normal horizontal incision. Women who require a classical Caesarean section are not able to safely have a normal vaginal delivery in the future.
Paediatric consultation before delivery is arranged when birth is anticipated and the estimated fetal weight is <2500 gm and/or any if fetal abnormalities are suspected on ultrasound. This consultation allows the paediatric staff to plan post-natal testing and follow up of the newborn if it is not admitted to the neonatal intensive care unit (NICU)
If a growth restricted baby needs to be admitted to a paediatric special care unit (birth weight 2000-2500 gm) or a NICU (birth weight <2000 gm), he/she will undergo tests to see why the baby was born small. In addition, the baby’s vision and hearing will be assessed. Most paediatric units, like Mount Sinai, have a developmental follow up program with a paediatrician that has expertise in infant development.
A small proportion of growth restricted babies may develop abnormalities of movement of the limbs (cerebral palsy), abnormal vision (retinopathy of prematurity), deafness, or may be slow to attain some of their developmental milestones. Well-planned resources to recognize these alterations from normal, together with interventions to assist the child and family, can make a great difference to childhood development and health.