The first trimester screening (or first trimester screen) typically
includes a maternal blood screening test and an ultrasound
Why Is the First Trimester Screen Done?
It's done to see if a fetus is at risk of having a chromosomal abnormality (such
as Down syndrome or
Edward syndrome) or birth
defect (such as heart problems).
It's important to remember that this is a screening test, not a diagnostic test.
If the test shows there might be a problem, another test must be done to confirm or
rule out a diagnosis.
Should I Have a First Trimester Screen?
The screen is considered safe, but it's your decision whether or not to have it.
The benefit of screening in the first trimester is more time for parents to prepare
for or address any health problems their baby may have.
When getting any screening, remember that there's a chance of false-positive or
False-positive results are when women whose results show high
risk for a chromosomal abnormality go on to have healthy babies.
False-negative results are when women whose results show low
risk have a baby with a chromosomal abnormality.
Talk to your doctor if you have questions about these screenings or your results.
What Happens During a First Trimester Screen?
This screening test ideally is done in two parts — a blood sample and an
The blood sample is taken with a finger prick or a regular blood draw. It measures
the levels of two proteins. Abnormal levels can mean that the baby has a higher-than-average
risk of a chromosomal abnormality.
During the ultrasound, the woman lies on her back while an ultrasound probe glides
across her lower belly. Sound waves bounce off the baby's bones and tissues to make
an image of the fetus. This can show the thickness of the space in the back of the
baby's neck. Extra fluid in the back of the neck may point to a higher risk of a chromosomal
Women whose results show a high risk for carrying a baby with a chromosomal abnormality
are offered diagnostic testing, usually through chorionic
villus sampling (CVS) in the first trimester or amniocentesis
in the second trimester.
Those who are not shown to be high risk in the first
trimester are still offered a second
trimester screening. That screening, which consists of more blood tests, checks
for chromosomal abnormalities and neural tube defects, and helps to confirm the findings
from the first trimester screen.
When Should I Have a First Trimester Screening?
The blood screening is usually done between 9 and 14 weeks. Women who also get
an ultrasound have one between 11 and 14 weeks.
When Are the Results Available?
Blood screening results usually are ready within a week or two. Ultrasound results
can be immediate.
When both the blood test and ultrasound are done, doctors usually calculate the
results together. This is called a combined first trimester screening.
Not all doctors calculate a woman's risk this way. Some wait until after a woman
has had other screenings in the second trimester. This is called an integrated
screening. Other practitioners don't do first trimester ultrasounds, instead
determining a woman's risk using the results of her first trimester and second trimester
blood screenings. This is called a serum integrated screening.
How your doctor calculates your results depends on your age, health risks, and
the services available at the provider's office.