If you’re pregnant, you’ve probably already had tons of tests. From tests where you have to pee in a cup to those where you’ll have a blood draw, pregnancy is a time of scrutiny about your body and its functions. In this post, we’ll discuss some of those blood tests, as well as dive into new research that suggests that it may be possible to predict a pregnancy complication, preeclampsia, with a blood test.
One of the most common tests you’ll have during pregnancy is a urinalysis. You pee in a cup, then the lab technicians check your urine for sugar (a sign of gestational diabetes), protein (a sign of preeclampsia), blood, and bacteria (which can indicate that you have a urinary tract infection). It’s likely that you’ll provide a sample for urinalysis at your first prenatal visit and then at other visits as needed.
The blood tests that you’re likely to see during pregnancy include a complete blood count, blood type and Rh factor, tests for HIV and other sexually transmitted infections, and cell-free fetal DNA testing, among others. A complete blood count or CBC is informative about whether or not you have iron-deficiency anemia (low iron), an infection, and how well your blood clots.
Blood type and Rh factor testing are essential both to determine your blood type in case you need a transfusion during labor, birth, or postpartum and to make sure that your and your baby’s blood are compatible. If your baby is positive for Rh factor (a protein that can hang out on the surface of red blood cells) and you are negative, your body recognizes the Rh factor as a foreign invader and makes antibodies to it. It’s not usually a problem in your first pregnancy, but during your baby’s birth or if you’ve had a miscarriage before, it’s possible for your blood to be exposed to baby’s, meaning that your immune system has time to mount a defense. This defense and the associated antibodies can be a problem in subsequent pregnancies. There’s a simple injection (RhoGAM) that you can receive that will interfere with your immune reaction, but you have to be tested to know that you need it.
Testing for sexually transmitted infections, including HIV, is essential during pregnancy because these infections can complicate your pregnancy and be passed to your baby. HIV, for instance, can cross the placenta and infect baby, but knowing that you are HIV positive means that your doctor or midwife will know that you need medicine that can help stop that transmission. Another sexually transmitted infection, gonorrhea, can result in blindness in your baby if you have it and aren’t treated for it during pregnancy.
The final blood test mentioned above, cell-free fetal DNA testing, requires a small amount of blood, in which clinicians and researchers can identify pieces of fetal DNA, the baby’s genetic material. It’s possible with this type of testing to tell whether or not there is a Y chromosome present and thus whether you’re having a boy or girl. It’s also possible to identify a risk for certain genetic conditions, including Down syndrome.
Researchers leveraged similar technology in a new study published in January 2022 in which they described methods to predict complications of pregnancy.  Morten Rasmussen, head of research at San Francisco biotech company Mirvie, and colleagues looked at cell-free RNA, another type of genetic material that your body’s cells use to make the things they need to survive, during pregnancy. They tracked 1,840 pregnancies and looked at more than 2,500 previously collected samples to show that cell-free RNA in women who develop preeclampsia looks distinctly different from the cell-free RNA in women who do not develop this complication. The research team also determined that cell-free RNA could predict preterm labor up to two months before that labor started.
In the future, the researchers will likely test whether the signals they found in cell-free RNA are predictive in larger groups of people that include more diversity. It’s possible that if their results are supported in bigger studies, this type of testing could be another blood test that is routinely offered to pregnant people.
- Rasmussen, Morten et al. “RNA profiles reveal signatures of future health and disease in pregnancy,” Nature, 2022.