Intrauterine Growth Restriction
(IUGR)Definition:
IUGR is defined as failure of the fetus to achieve its genetic growth potential. This usually results in a fetus that is small for gestational age (SGA) and babies born below a particular centile weight for gestation. (e.g. below the 3rd or 5th centile).This term (IUGR) implies that the fetus has been subjected to nutritional deprivation or to some cause of growth impairment so that its growth potential is not realized.
Small for gestational age (SGA): This term is referred to small fetuses that are merely constitutionally small and are otherwise perfectly normal. (SGA → some have IUGR and others are normal but small).
Causes of IUGR:
Chromosomal defects, e.g. trisomy 18, triploidy.
Single gene defects, e.g. Seckel`s syndrome.
Structural abnormalities, e.g. renal agenesis.
Infections, e.g. CMV, toxoplasmosis.
Maternal undernutrition, e.g. eating disorders, poverty.
Maternal hypoxia, e.g. altitude, cyanotic heart disease.
Drugs, e.g. cigarette smoke, alcohol, cocaine.
Placental factors:
Reduced utero-placental perfusion, e.g. inadequate trophoblast invasion, antiphospholipid syndrome, diabetes mellitus, sickle cell disease, multiple gestation, hypertension.
Reduced feto-placental perfusion, e.g. single umbilical artery, twin-twin transfusion syndrome.
Pathophysiology:
IUGR fetuses are frequently described as symmetric or asymmetric in terms of their body proportions.
Symmetrically small fetuses are usually associated with factors that directly impair the intrauterine growth potential of the fetus (i.e. chromosome abnormalities, viral infections), while asymmetric growth restriction is classically associated with utero-placental insufficiency.
The cause of fetal asymmetry follows upon the reduced oxygen transfer to the fetus and impaired excretion of CO2 by the placenta. The resulting fall in PO2 and rise in PCO2 in the fetal blood will induce a chemoreceptor response in the fetal carotid bodies with resulting vasodilatation in the fetal brain, myocardium and adrenal glands and vasoconstriction in the kidneys, splanchnic vessels, limbs and subcutaneous tissues. The liver circulation is also severely reduced; normally 50% of the well-oxygenated blood in the umbilical vein passes to the right atrium through the ductus venosus eventually to reach the fetal brain, with the remainder going to the portal circulation in the liver.
When there is fetal hypoxia, more of the well-oxygenated blood from the umbilical vein is diverted through the ductus venosus, which means that the liver receives less. The result of all these circulatory changes is an asymmetric fetus with relative brain sparing, reduced abdominal girth and skin thickness.
The vasoconstriction in the fetal kidneys results in impaired urine production and oligohydramnios. The fetal hypoxaemia also leads to severe metabolic changes in the fetus reflecting intrauterine starvation.
There will be reduced levels of nutrients, such as glucose and essential amino acids and hormones such as thyroxin and insulin. There are increased levels of corticosteroids and catecholamines, which reflect the increased perfusion of the adrenal gland. Haematological changes also reflect the chronic hypoxia with increased levels of erythropoietin and nucleated red blood cells.
The fetal hypoxia eventually leads to fetal acidaemia, both respiratory and metabolic, which if prolonged can lead to intrauterine death if the fetus is not removed from its hostile environment. IUGR fetuses are especially at risk from profound asphyxia in labour due to the further compromise of the utero-placental circulation caused by the uterine contractions.
Investigations:
The detection of an SGA infant contains two elements:Firstly, the accurate assessment of gestational age (early measurement of the fetal crown-rump length before 12wk. or the biparietal diameter between 12 and 20wk.). and
Secondly, the recognition of fetal smallness. The most precise method of detecting fetal smallness is by ultrasound biometry, in particular, measurement of the biparietal diameter, head circumference, abdominal circumference and femur length.
When a diagnosis of SGA has been made, the next step is to establish whether this represents IUGR or whether the fetus is "small normal".
A careful U/S scan of the fetal anatomy (for any fetal abnormalities to explain fetal smallness).
Features of utero-placental insufficiency would be as asymmetric fetus with a relatively small abdominal circumference, oligohydramnios and a high umbilical artery resistance.
Management:
The most important step is to make the diagnosis and then to identify a cause which may require treatment (like maternal hypertension). The mainstay of management is then serial monitoring with the aim of delivery as mature fetus as possible (thereby reducing the morbidity associated with prematurity), but in good condition, before hypoxic damage has occurred.Smoking, alcohol and drug abuse should be stopped.
The health of the woman should be maximized (optimize control of diabetes, thyroid dysfunction).
Bed rest in hospital is usually advised in an effort to maximize placental blood flow.
Clinical monitoring (maternal weight gain, uterine size or symphysis-fundal height, and fetal movement counting).
Cardiotocography or non-stress testing, biophysical profile, amniotic fluid volume, serial ultrasonic fetal biometry, and Doppler studies.
Serial ultrasound scans (for fetal biometry)are performed, cessation of fetal growth may be an indication in itself for delivery.
Doppler ultrasound, absence of blood flow in the umbilical artery during fetal cardiac diastole or reversed flow requires delivery as it reflects high placental resistance.
Occasionally, umbilical vein puncture under ultrasound guidance is used to obtain a fetal blood sample for blood gasses and acid-base status.
No effective drug therapy for IUGR has yet been found. (Aspirin, nitric oxide donors or anti-oxidants may be helpful in some cases).
A decision for vaginal or Caesarean delivery must be made. Frequently, the fetus will not be able to withstand the stresses of labour and an elective C/S is the method of choice.
If labour is induced, the fetus must be monitored very carefully.
Such babies should be delivered in units with the appropriate facilities and with expert neonatal care at hand.