USS Assessment of fetal anomalies

Question 1

Cystic hygroma can be noted on the 12 week nuchal scan; what are its associations?

Answer 1

• Second most common anomaly on 1st trimester scans
• 50% are associated with chromosomal abnormalities, particularly turners syndrome 45XO, downs, edwards and noonans syndrome.
• 50% occur in isolation and in the abscence of other abnormality may resolve spontaneously

Question 2

When can abdominal wall defects (omphalocele and gastroschisis) be accurately diagnosed on USS?

Answer 2

After 12 weeks.

Midgut herniation occurs around week 9-10 of embryogenesis and should return to the abdomen by 11+5

gastrochisis herniates lateral (right sided) to the umbilicus through an abdominal wall defect and can be diagnosed in the first trimester

Question 3

What is a normal NT measurement? What could an abnormal NT be associated with?

Answer 3

NT >/= 3.5mm is consider abnormal at 12 weeks - this increases proportionally with CRL and gestation

Increased NT is associated with:

• Chromosomal abnormality: Downs
• Cardiac defects
• Omphalocele
• Spinal defects
• Neonatal death
• Other genetic conditions: SMA, noonan syndrome, CAH

NT >6.5 has very poor prognosis:

• 20% risk fetal death (mostly before 20 wks)
• 45% risk significant abnormality even in abscence of normal serum screening

Question 4

What is the prevalence of major fetal structural abnormalities?

Answer 4

2-3%

Question 5

When can anencephaly be confidently diagnosed and what would the finding be?

Answer 5

11 weeks.
Abscence of cranial bone, with brain matter exposed to amniotic fluid - this will degenerate as the pregnancy progresses.

Question 6

When is the bladder visible and what abnormality may be seen?

Answer 6

14 weeks.
Absence after 14 weeks is abnormal.
Megacystitis - suggests posterior urethral valve or abscence of urethra. Can be associated with chromosomal abnormality and karyotype testing should be encouraged.