Everything You Need to Know About First Trimester Prenatal Tests
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The sheer number of first trimester prenatal tests now available to pregnant women can make your head spin. Tests offered in the first 11 weeks of a woman's pregnancy can vary widely and may depend on a host of factors including your age, ethnicity, family history, or your doctor.
To add to the confusion, many tests screen for certain conditions, but they are not diagnostic. This means they can predict whether you are at an increased risk of having a child with a disorder, but only a diagnostic test, which often involves a more invasive and therefore riskier procedure, can completely confirm or rule it out.
"It's kind of mind-boggling for people," says Mary Norton, M.D., a professor of obstetrics, gynecology, and reproductive sciences at the University of California, San Francisco. "I think the first thing that women should ask themselves is whether they want to have any testing. It can be a bit of a rabbit hole, and for some people, maybe they don't even want to get started."
She draws a clear distinction between routine prenatal tests like a urinalysis, which can uncover signs of treatable maternal diseases like gestational diabetes, and genetic tests, which can reveal a birth defect for which there is no treatment. "Those tests are worth searching your soul about," she says.
Following are some of the myriad tests you may receive in your first trimester.
Urine Test
Drink plenty of liquids before each prenatal visit, because you will always be asked to provide a urine sample. A urinalysis measures proteins in the urine that can indicate common complications in a pregnancy, such as gestational diabetes and preeclampsia. Testing positive for either condition with a urine test will lead to additional confirmatory testing.
Pap Smear
Typically performed on your first prenatal visit, a Pap smear tests for abnormal cervical cells and sexually transmitted infections (STIs). All ob-gyns perform this test, because giving birth with an undiagnosed and untreated STI may lead to your baby developing eye infections or pneumonia.
Blood Test
Be prepared to have several blood draws throughout your first trimester. Some of the following tests are routine, while others depend on your age, risk factors, and your doctor. You may be given the option, for example, to screen for chromosomal abnormalities in your growing baby—a decision that is personal to you.
Your initial blood test can measure for a lot of things, but here are the most common factors it measures.
• The level of hCG, a hormone produced by the fetus that is found in both your blood and your urine. Your ob-gyn can measure hCG levels to help determine your due date.
• Sexually transmitted infections, including HIV, syphilis, and hepatitis B.
• Disease immunity.
• Blood type, in the unlikely event you need a blood transfusion.
• Vitamin D deficiency.
• Rh factor incompatibility, which determines whether both you and your baby have a protein (Rh factor) carried by red blood cells. If you are incompatible, meaning you are Rh positive and your fetus is Rh negative, or vice versa, it may cause fetal anemia. This test helps your physician jump-start a treatment strategy involving a vaccine-like injection called a RhoGam shot.
• Blood sugar, if you are obese or have other risk factors for gestational diabetes.
• Genetic risk for everything from cystic fibrosis to sickle cell anemia, spinal muscular atrophy, among others.
Another blood test that is becoming increasingly routine in ob-gyn practices is noninvasive prenatal testing (NIPT), a blood test that screens for the most common chromosomal disorders, including Down syndrome, trisomy 13, and trisomy 18, along with sex chromosome abnormalities and microdeletions. It can be performed after nine weeks of pregnancy and is offered by genetic testing companies such as Genesis Serenity, Harmony, Natera, and MaterniT21.
Keep in mind that each company offers slightly different panels when they screen for certain conditions, and with varying accuracy.
"It's important to know that the vast majority of the data supporting the accuracy of NIPT is on Down syndrome," says Blair Stevens, a genetic counselor at the University of Texas Health Science Center in Houston. She says the microdeletion syndromes that some companies screen for have limited data. "The less data we have on these conditions, the less confident we can be on the accuracy of the results."
Many doctors recommend NIPT for women age 35 and over, who have had a child with a chromosomal disorder, or who have had an ultrasound showing a possible problem with the fetus. NIPT is also used to determine the sex of the baby. Because NIPT is not diagnostic, if a test comes back positive, then a more invasive test, such as an amniocentesis or a chorionic villus sampling (CVS) will be performed to confirm or rule out the disorder.
Imaging Test
A first trimester ultrasound is performed around 6 weeks and allows you to hear your baby's fetal heartbeat for the first time. Your doctor also performs an ultrasound to measure the fetus, which helps confirm your estimated due date, determine how many fetuses you're carrying, and to make sure the pregnancy is occurring in the uterus, and not in a fallopian tube, which would result in an ectopic pregnancy.
Nuchal translucency (NT) screening is a special type of ultrasound performed in the first trimester that measures the thickness of your fetus's nuchal fold, a spot of the base of the neck. Fetuses with increased fluid in the nuchal fold might have Down syndrome. NT screening results are combined with the results of other blood tests, including those that measure hCG levels, to improve its accuracy. If those combined results are high, then your physician may recommend you have a more invasive diagnostic procedure, such as chorionic villus sampling or amniocentesis.
Diagnostic Test
Chorionic villus sampling (CVS) can be performed between weeks 10 and 13 of your pregnancy. It is a diagnostic test that involves taking a small tissue sample from the placenta's chorionic villi—by inserting either a needle through your abdominal wall into your uterus, or a thin tube through your cervix and suctioning tissue from the placenta.
Examining the sample can reveal the genetic makeup of the fetus, including whether it has disorders such as Down syndrome, trisomy 13, trisomy 18, Tay-Sachs, or sickle cell anemia, among others. It cannot test for anatomical defects. CVS carries a small risk of miscarriage.
