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Maternal age
Nuchal translucency
Fetal heart rate
Serum biochemistry
New ultrasound markers

Screening for chromosomal defects

General principles

  • Every woman has a risk that her fetus/baby has a chromosomal defect.
  • The a priori risk depends on maternal age and gestation.
  • The patient-specific risk is calculated by multiplying the a priori risk with a series of likelihood ratios, which depend on the results of a series of screening tests.
  • The likelihood ratio for a given sonographic or biochemical measurement is calculated by dividing the percentage of chromosomally abnormal fetuses by the percentage of normal fetuses with that measurement.
  • Every time a test is carried out the a priori risk is multiplied by the likelihood ratio of the test to calculate a new risk, which then becomes the a priori risk for the next test.
  • If the tests are not independent of each other then more sophisticated techniques, involving multivariate statistics, can be used to calculate the combined likelihood ratio.