What 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.

How 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 second trimester.

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 specific risk.

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 of gestation.