2nd Trimester US

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While CRL is best during the first trimester, head measurement are best up to 20 weeks, with abdominal measurements being far less precise, but good for identifying pathology. Femur length is best during third trimester.

If a gestational age was determined during 1st trimester, it should not be changed based on data a later ultrasound (i.e. the 20 week US).

Quadruple Test: for Trisomy 21 performed between 15 and 22 weeks

  • AFP: Decreased in Trisomy 21, increased in neural tube defects, gastroschisis, fetal-maternal hemorrhage, maternal hepatitis, maternal HCC
  • Estriol: Decreased in Trisomy 21
  • HCG: Increased in Trisomy 21
  • Inhibin A: Increased in Trisomy 21

Cervical incompetence

Funneling is when internal os is open
Cervical length should be 3 cm



Essentially constant between 14-38 weeks, upper limit of normal is 10 mm
Hydrocephalus caused my aquaductal stenosis (20%), hemorrhage, Chiari II, Dandy Walker, etc

Cisterna Magna: 2-10 mm

Chorid Plexus Cysts

Can be normal, found in 0.2-2.5%
Can be a/w trisomy 18 and 21
Correlate with triple screen, maternal age and additional findings

Cerebellar vermis: open until 18 weeks

Nuchal Thickness

No greater than 6 mm when measured between 16-22 weeks
a/w chromosomal abnormailities
Possibly secondary to cardiac defects or lymphatic obstruction




Minimal pyelectasis is normal, but caliectasis or ureterectasis is not normal
Before 23 weeks, pelvis can measure up to 4 mm
After 23 weeks, pelvis can measure up to 7-10 mm
Differential includes reflux, obstruction by Ureterocele, UPJ obstruction, urethral aresia, Posterior urethral valves, Eagle-Barrett (Prune Belly) syndrome of hypoplasia of abdominal musculature, dilated renal collecting system and cryptorchidism