Foetal Growth Centile Calculator

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What is the difference between SGA and IUGR?

SGA (Small for Gestational Age) is a statistical description: EFW or AC below the 10th centile for gestational age. Some SGA fetuses are constitutionally small (normal variant) and not at increased risk. IUGR (Intrauterine Growth Restriction) implies a pathological process — usually placental insufficiency — that prevents the fetus from reaching its genetic growth potential, evidenced by abnormal Dopplers, oligohydramnios, or growth velocity decline. IUGR carries higher perinatal morbidity and mortality than isolated SGA.

What causes IUGR?

The most common cause is uteroplacental insufficiency, resulting in impaired oxygen and nutrient transfer to the fetus. Risk factors include pre-eclampsia, chronic hypertension, diabetes (with vascular disease), thrombophilias, severe maternal anaemia, heavy smoking, cocaine use, multiple pregnancy, uterine anomalies, and fetal chromosomal or structural abnormalities. In approximately 40% of cases, the cause is not identified (idiopathic).

What is the umbilical artery Doppler and why does it matter?

The umbilical artery Doppler measures resistance to blood flow in the placenta. In a healthy placenta, diastolic blood flow is maintained throughout the cardiac cycle. As placental resistance increases (due to progressive obliteration of placental vasculature), diastolic flow first diminishes, then is absent (AREDF), and eventually reverses. AREDF is associated with a 50–100x increase in perinatal mortality compared to normal Doppler and triggers intensive management.

What are antenatal corticosteroids and why are they given?

Antenatal corticosteroids (betamethasone 12 mg IM x2 doses 24 hours apart, or dexamethasone) accelerate fetal lung maturity, reducing the risk of respiratory distress syndrome (RDS) by approximately 50% and intraventricular haemorrhage by 40% when given before preterm delivery. They are recommended between 24+0 and 34+6 weeks gestation when delivery is anticipated within 7 days. A rescue course can be given after 7 days if delivery is still anticipated preterm.

What is the Delphi IUGR classification?

The Delphi consensus (Gordijn et al, 2016) defines IUGR as either: a fetus with EFW or AC <3rd centile; or EFW/AC <10th centile combined with absent end-diastolic flow in the umbilical artery or CPR <5th centile or uterine artery PI >95th centile. Four stages of severity (Stage I–IV) guide delivery timing: Stage I can be managed conservatively to term; Stage II–III requires intensive monitoring with delivery based on Doppler deterioration; Stage IV requires immediate delivery.

What is the cerebroplacental ratio (CPR)?

The CPR is the ratio of middle cerebral artery pulsatility index (MCA PI) to umbilical artery pulsatility index (UA PI). A CPR above 1.0 is normal. When CPR falls below 1.0, the fetus is redistributing blood flow to protect the brain — a sign of circulatory compromise even when individual Doppler values are within normal limits. Low CPR is particularly useful in detecting late-onset IUGR where UA Doppler alone may appear normal.

When should IUGR fetuses be delivered?

Delivery timing depends on IUGR severity: Severe early-onset IUGR with AREDF: deliver at 32–34 weeks after corticosteroids; with reversed DV a-wave: deliver immediately regardless of gestation if viable. Moderate IUGR: based on weekly Doppler and CTG, typically 34–37 weeks. Mild late-onset SGA with low CPR: 37–38 weeks. All decisions must balance prematurity risks against ongoing intrauterine compromise risk.

What are the long-term outcomes for IUGR babies?

IUGR is associated with increased risks in later life including hypertension, insulin resistance, type 2 diabetes, cardiovascular disease, and obesity — the Barker hypothesis or 'Developmental Origins of Health and Disease' (DOHaD). These risks are amplified by postnatal catch-up growth. Neurodevelopmental outcomes depend heavily on gestational age at delivery and NICU care quality. Long-term follow-up in specialist neonatal/developmental paediatric clinics is recommended for severely IUGR infants.