Case 69: Antenatal ultrasound

 

Case 69: Antenatal ultrasound

CASE 69: ANTENATAL ULTRASOUND

History

A 31-year-old woman attends a routine antenatal appointment at 28 weeks’ gestation. She is

para 2 having had two term vaginal deliveries previously, with the same partner. Her children

are aged 4 and 2 years and weighed 3.7 and 3.6 kg respectively at birth. She reports no antenatal concerns, there is no abdominal pain, no vaginal bleeding and good fetal movements

are reported.

Initial antenatal booking blood tests were normal, as were the first-trimester nuchal screen

and 20-week anomaly scans.

She is a non-smoker and has abstained from alcohol during the pregnancy.

Examination

The woman appears well with no signs of oedema. Her BMI is 24. Blood pressure is 115/74 mmHg.

The symphysiofundal height is 24 cm. The fetus is felt to be a cephalic presentation.

Auscultation with hand-held Doppler confirms the fetal heartbeat to be 150/min.

INVESTIGATIONS

Urinalysis: negative.

Ultrasound: biparietal diameter and femur length are on the 10th centile. Abdominal

circumference and estimated fetal weight are below the fifth centile. The liquor volume

is normal. Umbilical artery resistance index is within normal range.

Questions

• What are the possible causes of the small fetal size and what further investigations

would you propose?

• How would you manage this pregnancy from now?100 Cases in Obstetrics and Gynaecology

190

ANSWER 69

Intrauterine growth restriction is defined as predicted birth weight less than 10th centile for

gestational age. The most common cause is uteroplacental insufficiency, commonly part of

the pre-eclampsia process. However there are multiple alternative causes.

Causes of small for gestational age fetus

Maternal Fetal

Chronic hypertension Maternal smoking Congenital infection

Pre-eclampsia Excess alcohol Chromosomal anomaly

Diabetes Malnutrition Constitutionally small fetus

Chronic renal disease Hypoxic lung disease

In this case, there is no evidence of hypertension or pre-eclampsia and as this is the same partner

and only 2 years since her last birth, it would be unusual for the woman to develop pre-eclampsia

in this pregnancy. She has no personal or family history of diabetes, renal or respiratory disease.

It is therefore likely that the fetus is small due to fetal rather than maternal factors.

Further investigation

The baby may be constitutionally small but in light of the previous two babies being of normal

birth weight, this is less likely.

A TORCH screen should be performed to identify maternal viral infection that may be associated with fetal growth restriction. This includes toxoplasmosis, rubella (if non-immune

at booking), cytomegalovirus and herpes simplex. Other infections may be associated and

include Epstein-Barr virus, syphilis, hepatitis, parvovirus and HIV.

The woman should be offered amniocentesis to identify any chromosomal anomaly. These

include trisomies 21, 18 and 13 and monosomy X (Turner’s syndrome).

Management of the pregnancy

The management depends on the investigation results. If the chromosomal analysis is abnormal then the woman should be able to discuss her options for continuing the pregnancy, as

trisomy 18 and 13 are incompatible with postnatal survival. If the infection screen reveals a

recent maternal infection then the management involves close fetal medicine surveillance to

determine appropriate timing of delivery and neonatal treatment.

If no cause for the growth restriction is found then serial ultrasound scans should be performed. Reduced amniotic fluid volume followed by increased resistance in the umbilical

arteries is associated with adverse perinatal outcome in a growth-restricted fetus and can be

used to determine timing and mode of delivery. Where preterm delivery is indicated before

36 weeks’ gestation, maternal corticosteroid injections should be administered to reduce the

incidence of respiratory distress syndrome in the neonate.

KEY POINTS

• Intrauterine growth restriction is caused most commonly by uteroplacental

insufficiency.

• Serial growth scans should be carried out if IUGR is suspected.

• Tests for infection or chromosomal anomaly may be indicated for IUGR pregnancies where no other obvious cause is apparent