Prenatal labs are a set of blood tests, urine tests, and swabs performed at various points during pregnancy to check your health and your baby’s health. Most are done at your first prenatal visit, typically between 8 and 12 weeks, while others happen at specific points later in pregnancy. Together, they screen for infections that could pass to your baby, blood conditions like anemia, blood sugar problems, and genetic conditions.
The First Prenatal Blood Panel
Your first prenatal visit usually involves drawing several tubes of blood at once. This single blood draw covers multiple tests bundled into what’s called a prenatal panel. Here’s what it typically includes:
- Complete blood count (CBC): Measures your red blood cells, white blood cells, and platelets. It’s mainly used to check for anemia, which is common in pregnancy. You’re considered anemic if your blood volume percentage of red blood cells drops below 33% in the first and third trimesters, or below 32% in the second trimester.
- Blood type and Rh factor: Determines whether your blood is type A, B, AB, or O, and whether you’re Rh-positive or Rh-negative. Rh factor is a protein on the surface of red blood cells. If you lack it (Rh-negative) and your baby has it (Rh-positive), your immune system could attack the baby’s blood cells in this or future pregnancies.
- Rubella immunity: Checks whether you have antibodies against rubella (German measles). Most people were vaccinated as children, but immunity can fade over time. Rubella during pregnancy can cause serious birth defects.
- Hepatitis B: A routine screen for all pregnant patients. Hepatitis B can pass to the baby during delivery, but treatment at birth can prevent infection.
- Syphilis and chlamydia: Both are tested early in pregnancy. Gonorrhea testing may be added depending on your risk factors. These infections are treatable but can cause complications for the baby if left undiagnosed.
- HIV: Offered to all pregnant patients, and required by law in some states. If HIV is detected early and treated, the chance of passing it to the baby drops dramatically.
The rationale behind testing for all these infections is straightforward: early detection allows treatment that can prevent the infection from reaching the baby. Most of these conditions are either treatable or manageable when caught before delivery.
Urine Tests Throughout Pregnancy
You’ll give a urine sample at most prenatal visits. Providers check for two things in particular: protein and glucose.
Protein in urine is one of the most commonly performed prenatal screening tests. During pregnancy, it’s traditionally been a hallmark sign of preeclampsia, a dangerous blood pressure condition that can develop after 20 weeks. However, protein can also show up for less alarming reasons. Patients who are older, carrying twins or multiples, or have a higher BMI sometimes exceed protein thresholds without having any underlying disease. If you have a preexisting condition like chronic high blood pressure, diabetes, or an autoimmune disorder, your provider will likely check your protein levels at your first visit to establish a baseline for comparison later.
Glucose in urine can be an early signal of blood sugar issues, though the definitive test for gestational diabetes comes later.
Gestational Diabetes Screening
Between 24 and 28 weeks, you’ll take a glucose challenge test. You drink a sugary liquid, wait one hour, and then have your blood drawn. A blood sugar level below 140 mg/dL is considered normal. Some clinics use a lower cutoff of 130 mg/dL.
If your result falls between 140 and 190 mg/dL, you’ll be asked to come back for a longer three-hour glucose tolerance test to confirm whether you have gestational diabetes. A result of 190 mg/dL or higher on the initial one-hour test is enough to diagnose gestational diabetes without the follow-up.
What Happens If You’re Rh-Negative
About 15% of people are Rh-negative, meaning their red blood cells lack the Rh protein. This only becomes a problem if the baby is Rh-positive, because the mother’s body can produce antibodies that attack the baby’s blood cells.
To prevent this, Rh-negative patients receive an injection of Rh immunoglobulin (commonly known by the brand name RhoGAM) at 26 to 28 weeks of pregnancy. If the baby is born Rh-positive, a second dose is given within 72 hours of delivery. The injection is also given within 72 hours after any event that could mix maternal and fetal blood, such as a miscarriage, amniocentesis, or abdominal injury. If more than 12 weeks pass after a dose without delivery, a repeat dose is recommended.
Genetic Screening Options
Genetic screening during pregnancy falls into two categories: screening for conditions your baby might have, and carrier screening for conditions you might unknowingly carry in your genes.
Screening for Chromosomal Conditions
These tests check for conditions like Down syndrome, trisomy 18, and trisomy 13. ACOG recommends that all pregnant patients be offered genetic screening regardless of age or risk level. There are two main approaches.
Traditional serum screening uses a blood draw from the mother, sometimes paired with an ultrasound measurement of fluid at the back of the baby’s neck (nuchal translucency). This is typically done in the first or second trimester. Cell-free DNA screening, also called NIPT (non-invasive prenatal testing), analyzes fragments of the baby’s DNA circulating in the mother’s blood. It’s the most sensitive and specific screening test for common chromosomal conditions, though it is still a screening test, not a definitive diagnosis. A positive result on either type of screening is confirmed through amniocentesis or chorionic villus sampling.
Carrier Screening
Carrier screening checks whether you carry a gene for a condition you could pass to your baby, even if you’re healthy yourself. Two conditions are recommended for universal screening: cystic fibrosis and spinal muscular atrophy. Both should be offered to anyone who is pregnant or considering pregnancy.
Tay-Sachs disease screening is recommended specifically for people of Ashkenazi Jewish, French-Canadian, or Cajun descent, or anyone with a family history of the condition. You can decline any or all carrier screening, and you can also request testing for specific conditions even if they aren’t part of the standard panel for your background.
Group B Strep Screening
In the 36th or 37th week of pregnancy, your provider will swab your vagina and rectum to check for Group B streptococcus (GBS) bacteria. About 1 in 4 pregnant people carry GBS, which is harmless to adults but can cause serious infections in newborns during delivery. If you test positive, you’ll receive antibiotics through an IV during labor to protect the baby. The test is repeated with each pregnancy because GBS colonization can come and go.
Thyroid Testing
Unlike the tests above, thyroid screening is not routine during pregnancy. ACOG specifically recommends against universal thyroid screening for pregnant patients. However, if you have a history of thyroid problems, symptoms of thyroid disease, or other risk factors, your provider will check your thyroid levels. During pregnancy, the normal range for thyroid-stimulating hormone shifts: it should stay below 2.5 in the first trimester and below 3.0 afterward.
Timeline of Prenatal Labs
Keeping track of which tests happen when can feel overwhelming. Here’s a rough sequence:
- First visit (8 to 12 weeks): Complete blood count, blood type and Rh factor, rubella immunity, hepatitis B, HIV, syphilis, chlamydia, urinalysis. Carrier screening and early genetic screening (NIPT or first-trimester serum screen) are also offered around this time.
- 15 to 22 weeks: Second-trimester serum screening, if not already completed through first-trimester testing or NIPT.
- 24 to 28 weeks: Glucose challenge test for gestational diabetes. Repeat CBC to recheck for anemia. RhoGAM injection for Rh-negative patients.
- 36 to 37 weeks: Group B strep swab.
Some infections, including syphilis and HIV, may be retested in the third trimester depending on your risk factors or state requirements. Urine screening continues at nearly every visit throughout pregnancy.