What Moms-to-Be Need to Know About Pregnancy Blood Tests

Today, obstetricians have many tools at their disposal to learn important information about the health of a baby. Most people are familiar with ultrasounds, in which a technician runs a wand over the belly to pick up high-frequency sound waves and make an image of the developing baby.

But moms-to-be might be surprised that much of the information about their baby’s health can come from a simple blood test (of the mother, not the baby). Pregnancy involves a lot of bloodwork, some of it standard and some recommended based on certain risk factors.

Each mother and her doctor will have choices to make about which blood tests to have and which to skip, says Dawn Trollip, MD, an obstetrician who delivers babies at UNC REX Healthcare.

“I counsel my patients that 3 to 5 percent of babies are born with a physical or mental disability, and not all birth defects are detectable through current available testing,” she says. “Women need to think of their own values and preferences when deciding whether to test.”

Here’s what pregnant women need to know.

Standard Blood Tests in Pregnancy

Some blood tests are standard for virtually every pregnancy, Dr. Trollip says. These include testing to:

  • Screen for anemia, a condition marked by too few red blood cells.
  • Check blood type, in the unlikely event that a blood transfusion is needed during delivery or the postpartum period.
  • Screen for antibodies that could harm the fetus. For example, some moms with Rh negative blood types build up antibodies to attack Rh positive blood types, which could be problematic if their baby has Rh positive blood.
  • Screen for sickle cell trait. If you and your partner have sickle cell trait, your baby has a 1 in 4 chance of having sickle cell disease.
  • Check for rubella and varicella (chickenpox) immunity because of previous infection or vaccination. Both of these conditions can harm a fetus, and pregnant women cannot be vaccinated for them. Women without immunity should stay away from any potentially infected people and get immunized as soon as they deliver.
  • Test for sexually transmitted infections, including HIV, syphilis, hepatitis, gonorrhea and chlamydia. If the mother is infected, doctors can take steps to protect the baby from infection.
  • Screen for gestational diabetes, a temporary diabetes that can cause pregnancy complications.

Testing for Chromosomal and Neural Tube Abnormalities

Some women elect to be screened for aneuploidy, which means the baby has too many or too few chromosomes. Most commonly, aneuploidy results in trisomy 21 (Down syndrome). Rarely, the baby can have the more severe and life-threatening abnormalities of trisomy 18 or trisomy 13.

Aneuploidy screening can be done in multiple ways:

  • First-trimester screening: This two-step process uses a blood test that checks for levels of pregnancy-specific substances and an ultrasound that shows the fluid behind the baby’s neck (nuchal translucency) to determine a woman’s risk of having a baby with a chromosomal abnormality. This is a screening, not a diagnostic test, and is up to 90 percent accurate, Dr. Trollip says.
  • Second-trimester screening: A “quadruple” blood test, sometimes called “the QUAD screen,” is used to measure four different substances in the woman’s blood. It screens for Down syndrome, trisomy 18 and neural tube defects (problems with the brain, spine or spinal cord).
  • Cell-free DNA: This blood test, sometimes called noninvasive prenatal screening, provides information about the DNA of the baby without actually testing the baby or the amniotic fluid. Fragments of fetal DNA naturally escape the placenta and enter the mom’s bloodstream, where they can be screened after about 10 weeks gestation. A “low-risk” result is more than 99 percent accurate in screening for Down syndrome and in determining the baby’s gender, Dr. Trollip says. This method also has a much lower false positive risk (less than 1 percent) than first- and second-trimester screening (approximately 5 percent).

If one of the screening tests shows a concern, the mother can opt for a detailed follow-up ultrasound or an invasive diagnostic test, such as chorionic villus sampling or amniocentesis. Either way, she will see a maternal-fetal medicine specialist, who specializes in high-risk pregnancies.

Any woman can opt for chromosome testing, Dr. Trollip says, but it’s especially recommended for women older than age 35 and women who have had a baby with a birth defect previously. Both those groups are at higher risk of having a baby with a chromosomal abnormality.

“If someone tests within normal limits, it’s not a guarantee, but it’s highly reassuring,” Dr. Trollip says. “That said, we don’t have any tests that can detect all genetic abnormalities.”

Some mothers also choose to have an alpha-fetoprotein (AFP) test, which checks for the levels of AFP in the mother’s blood. Too much of the protein could indicate a neural tube defect such as spina bifida, when the spinal cord doesn’t close completely. Too little could indicate Down syndrome.

Genetic Carrier Screening

Unlike chromosome testing, genetic carrier screening is a test of the mother—and sometimes the father—and not the fetus.

Genetic diseases that can be passed down include cystic fibrosis, spinal muscular atrophy and Tay-Sachs disease. Parents can be carriers and not know it; if both parents carry the gene mutation, the baby has a 25 percent chance of having the disease and a 50 percent chance of being a carrier but not having the disease. There is a 25 percent chance of the best scenario: The baby will not be a carrier or have the disease.

Typically, the mother is screened first and then the father is screened if the mother is a carrier for any conditions.

Genetic carrier screening only has to be done once in a parent’s lifetime, even if you have multiple children, and can be done even before conception. If both parents are carriers for the same genetic disorder, the baby can be tested in utero to see if he or she is affected.

Deciding Whether to Get Chromosome or Genetic Testing

Each woman has to decide for herself whether she and her partner want all the potentially available information about their baby’s health during her pregnancy.

“It’s a lot of information, and the question is, does the benefit of knowing this information outweigh the anxiety of waiting for it?” Dr. Trollip says, noting that the test results typically take about seven days.

Some women choose not to get any testing, and others want to know everything they can for various reasons, she says. In cases where the screening test delivers potentially bad news, a diagnostic test is recommended.

“It’s important to think about why you want the screening tests and what you would do with the results, and then you could decide if it’s right for you,” Dr. Trollip says. “This information can be helpful—for example, some people want to prepare for extra care that a child might need—or it can be a burden; what matters is how you feel about it.”

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