Prenatal Test: First Trimester Screening
What Is the First Trimester Screening?
The first trimester screening (or first trimester screen) typically includes a maternal blood screening test and an ultrasound exam.
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-negative results:
- 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 ultrasound exam:
- 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 disorder.
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.