is involved in early pregnancy screening?
A maternal blood sample is obtained for evaluation of biochemical
assays. Maternal serum levels of beta-human chorionic gonadotropin
(hCG) and pregnancy associated plasma protein-A (PAPP-A)
are quantified. A fetal ultrasound is performed for assessment
of nuchal translucency (NT) diameter and crown rump length
(CRL) by personnel who are certified to interpret nuchal
translucency measurement. Centers providing this service
should undergo regular peer review quality assurance and
have sufficient volume of cases and physician supervision
to confirm the validity of such testing.
accurate is early pregnancy screening?
First trimester screening assesses risk of only two chromosomal
or karyotype abnormalities, trisomy 21 and trisomy 18. Detection
rates for these chromosomal abnormalities are greater than
75% to 80% in most studies, and higher than detection rates
with second trimester maternal serum screening. Unlike second
trimester maternal serum screening, assessment of fetal
open neural tube defect (ONTD) or spina bifida risk can
not be obtained with first trimester screening. Screening
for ONTD requires additional maternal serum testing in the
Who should consider
first trimester screening?
First trimester screening is available for women of any
age but is designed to increase detection of fetuses with
these specific chromosomal abnormalities in primarily low
risk individuals with singleton gestations. Women who have
additional risk factors such as a positive family history
or who are older than 34 years of age (advanced maternal
age) should undergo genetic counseling, and should be offered
consideration of diagnostic testing such as chorionic villus
sampling (CVS) or genetic amniocentesis. If such at risk
patients elect not to undergo diagnostic testing, first
trimester screening may be offered to further assess pregnancy
How are results interpreted?
What follow-up testing is needed?
Every screening result will assign a numerical risk of having
an affected fetus, including a separate risk for trisomy
21 (Down syndrome) and trisomy 18. If the first trimester
screening result is normal or negative, additional testing
should include second trimester maternal serum screening
for ONTD which is generally obtained at approximately 16
weeks of gestation.
Detailed ultrasound assessment
for fetal congenital abnormalities is recommended at 18-20
weeks of gestation. A negative screen result does not exclude
the diagnosis of trisomy 21 or trisomy 18.
An abnormal or positive first
trimester screen indicates either an increased risk for
trisomy 21 or trisomy18. Abnormal screening results are
not diagnostic and therefore not conclusive of a fetal abnormality.
Abnormal screening results are also associated with other
fetal abnormalities including other chromosomal abnormalities
and other congenital malformations not related to a chromosomal
abnormality but which might not be detected until later
in the second trimester.
Genetic counseling is indicated
when an abnormal result is obtained. Options that should
be considered include diagnostic testing such as CVS or
genetic amniocentesis which will definitively assess fetal
karyotype, and detailed evaluation of fetal anatomy with
ultrasound. If CVS testing is performed and results are
normal, additional testing should include second trimester
maternal serum screening for ONTD and detailed ultrasound
assessment for fetal congenital abnormalities at 18-20 weeks