Antenatal screening is a collection of tests offered during pregnancy to check for health conditions in both the pregnant person and the developing baby. These tests estimate the likelihood of conditions like Down syndrome, gestational diabetes, and certain infections, often months before any symptoms appear. Most antenatal screening is non-invasive, involving blood draws and ultrasounds rather than procedures that carry risk to the pregnancy.
One important distinction: screening tests tell you about risk, not certainty. A screening result might show a higher or lower chance of a particular condition, but only a diagnostic test can give a definitive yes or no. If a screening result comes back as high-risk, you’ll typically be offered a follow-up diagnostic test to confirm or rule out the condition.
First Trimester Screening (Weeks 11 to 13)
The earliest round of screening happens between weeks 11 and 13 and focuses on chromosomal conditions, primarily Down syndrome (trisomy 21), trisomy 18, and trisomy 13. This “combined test” has two parts. A blood draw measures levels of three chemicals in your blood: a pregnancy hormone (hCG), a protein produced by the placenta (PAPP-A), and alpha-fetoprotein (AFP). Levels that are higher or lower than average can signal an increased chance of one of these conditions.
The second part is an ultrasound that measures a small pocket of fluid at the back of the baby’s neck, called the nuchal translucency. A larger-than-expected measurement can indicate a chromosomal abnormality or a heart defect. Your care team combines the blood results, the ultrasound measurement, and your age to calculate an overall risk score.
Cell-Free DNA Screening (NIPT)
A newer option called non-invasive prenatal testing, or NIPT, analyzes tiny fragments of the baby’s DNA circulating in your bloodstream. It can be done as early as 10 weeks and screens for the same major trisomies as the combined test, but with significantly higher accuracy. For trisomy 21 in high-risk pregnancies, NIPT has a positive predictive value around 97.5%, meaning that when the test flags a high risk, it’s correct the vast majority of the time. In lower-risk pregnancies, that figure drops to about 86%, which still outperforms traditional blood screening.
For NIPT to work, enough of the baby’s DNA needs to be present in your blood sample. Labs generally require a fetal DNA fraction of at least 2 to 4%. Factors like early gestational age and higher body weight can lower that fraction, occasionally requiring a repeat blood draw. Despite its high accuracy, NIPT is still a screening test. A positive result is typically followed by amniocentesis or chorionic villus sampling for confirmation.
Second Trimester Blood Screening
Between weeks 15 and 20, you may be offered a second blood test called the quad screen. This measures four substances: AFP, hCG, a form of estrogen called estriol, and a hormone called inhibin-A. The quad screen estimates risk for Down syndrome, trisomy 18, and neural tube defects like spina bifida. Some providers use it as a standalone test, especially if you missed the first trimester window. Others combine results from both trimesters into an integrated screen for better overall accuracy.
The 20-Week Anatomy Scan
Around the midpoint of pregnancy, a detailed ultrasound surveys the baby’s physical development. This is often the most thorough imaging you’ll receive, and the sonographer works through a long checklist: the brain, heart, spine, kidneys, lungs, stomach, intestines, limbs, hands, feet, and facial features including the lips, nose, and eyes. They also record the heart rate, check blood flow through the umbilical cord, measure amniotic fluid levels, and note the position of the placenta.
This scan can detect a range of structural conditions, including congenital heart defects, spina bifida, cleft lip, skeletal abnormalities, and abdominal wall defects. It also provides another opportunity to look for markers of chromosomal conditions. Not every abnormality is visible on ultrasound, and some findings turn out to be minor or resolve on their own. When something concerning does appear, you’ll be referred for further imaging or diagnostic testing.
Gestational Diabetes Screening
Between 24 and 28 weeks, nearly all pregnant people are screened for gestational diabetes. In the most common approach used in the U.S., you drink a sugary solution containing 50 grams of glucose, and your blood sugar is checked one hour later. If that number comes back above a certain threshold, you move on to a longer, more precise test.
The follow-up is an oral glucose tolerance test. After fasting overnight, your blood sugar is measured, then you drink a larger glucose solution (either 75 or 100 grams depending on the protocol), and your levels are checked at one, two, and sometimes three hours. A diagnosis of gestational diabetes is made when multiple readings exceed specific cutoffs. For example, under commonly used criteria, a fasting level at or above 95 mg/dL combined with a one-hour reading at or above 180 mg/dL would meet the threshold. Early detection matters because gestational diabetes can usually be managed with dietary changes, exercise, and monitoring, reducing the risk of complications for both you and the baby.
Blood Type and Rhesus Factor
At your first prenatal visit, a blood draw checks your blood type and Rhesus (Rh) factor. About 15% of people are Rh-negative, which becomes relevant if the baby is Rh-positive. In that situation, your immune system can treat the baby’s blood cells as foreign and produce antibodies against them. This rarely causes problems in a first pregnancy, but it can lead to serious complications in subsequent pregnancies if untreated.
If you’re Rh-negative, your antibody levels are rechecked between 24 and 28 weeks. Assuming no antibodies have developed, you receive an injection of Rh immunoglobulin around that time. A second dose is given after delivery if the baby turns out to be Rh-positive, ideally within 72 hours. This treatment prevents your body from building the immune response that could affect a future pregnancy.
Infectious Disease Screening
Early prenatal blood work includes testing for several infections that can be passed to the baby during pregnancy or delivery. The CDC recommends screening all pregnant people for HIV, hepatitis B, hepatitis C, and syphilis at the first prenatal visit. Screening for chlamydia and gonorrhea is recommended for everyone under 25 and for older individuals with risk factors. Tuberculosis testing is offered to those at higher risk.
Many of these infections cause no obvious symptoms but can have serious consequences for the baby if left untreated. Syphilis, for instance, can lead to premature birth, low birth weight, or stillbirth. HIV can be transmitted during delivery but the risk drops dramatically with treatment. Identifying these infections early gives time to start treatment that protects both you and the baby well before delivery.
Group B Strep Screening
Later in pregnancy, at 36 or 37 weeks, you’ll be screened for group B streptococcus (GBS), a type of bacteria that naturally lives in the digestive and reproductive tracts of about 25% of healthy adults. It’s harmless to you but can cause serious infections in newborns if passed along during delivery.
The test is simple: a swab of the vagina and rectum is sent to a lab. This screening happens during every pregnancy, even if you’ve tested negative before, because GBS colonization comes and goes. It’s also done when a cesarean birth is planned. If you test positive, you receive antibiotics through an IV during labor, which significantly lowers the chance of the baby developing a GBS infection.
Screening vs. Diagnostic Tests
Throughout pregnancy, it helps to keep the distinction between screening and diagnostic testing clear. Screening tests like blood panels, NIPT, and standard ultrasounds are low-risk and widely offered. They sort pregnancies into lower-risk and higher-risk categories. Diagnostic tests like amniocentesis and chorionic villus sampling are more invasive, carrying a small risk of complications, but they provide definitive answers. You’re never required to have either type. The purpose of screening is to give you information so you can make informed choices about what comes next.