Antenatal screening is a set of tests offered during pregnancy to check for health conditions in the baby and the pregnant person. These tests look for chromosomal conditions like Down syndrome, physical abnormalities in the baby’s developing organs, infections that could pass from parent to child, and conditions like gestational diabetes. Screening tests estimate risk rather than give a definitive yes or no answer, and they’re offered at specific points throughout pregnancy.
How Screening Differs From Diagnostic Testing
The distinction matters. A screening test tells you whether your baby has a higher or lower chance of having a particular condition. It flags who might benefit from further investigation. A positive screening result does not mean your baby has the condition. It means additional testing is recommended.
Diagnostic tests, by contrast, can confirm or rule out a condition with greater than 99.9% accuracy. The two main diagnostic procedures are chorionic villus sampling (CVS), performed between 10.5 and 13.5 weeks, and amniocentesis, performed from 15 weeks onward. CVS involves taking a small tissue sample from the placenta, while amniocentesis draws a small amount of fluid from around the baby. Both carry a slight risk of miscarriage: up to 1% for CVS and less than 1% for amniocentesis. Because of that risk, diagnostic tests are typically offered only after a screening test suggests a higher chance of a problem.
First Trimester Screening
Between 10 and 13 weeks of pregnancy, you may be offered what’s often called the combined test. It has two parts: an ultrasound and a blood draw. The ultrasound measures the fluid at the back of the baby’s neck, known as nuchal translucency. A larger than expected measurement can indicate a higher chance of chromosomal conditions. The blood test checks levels of two substances: a pregnancy hormone produced by the placenta and a protein associated with early pregnancy. Together, these results are combined with your age to calculate a risk score for conditions like Down syndrome (trisomy 21) and Edwards’ syndrome (trisomy 18).
Second Trimester Screening
Between 15 and 22 weeks, a blood test called the quadruple screen may be offered. It measures four substances: a protein produced by the baby, a hormone from the placenta, a form of estrogen produced by both the baby and the placenta, and another placental hormone. The test is most accurate when done between 16 and 18 weeks.
Results help estimate the risk of Down syndrome, Edwards’ syndrome, and neural tube defects, which are conditions affecting the brain or spinal cord such as spina bifida. As with first trimester screening, a higher-risk result doesn’t confirm a diagnosis. It identifies pregnancies where further testing would be worthwhile.
Cell-Free DNA Screening (NIPT)
A newer option, sometimes called non-invasive prenatal testing or NIPT, analyzes fragments of the baby’s DNA circulating in the pregnant person’s blood. It can be done in any trimester and screens for specific chromosomal conditions, primarily trisomy 21 (Down syndrome), trisomy 18 (Edwards’ syndrome), and trisomy 13 (Patau’s syndrome).
NIPT is considerably more accurate than traditional blood-based screening. In a study of over 36,000 pregnancies, detection rates reached 99% or higher for Down syndrome, with a very low false-positive rate. For Edwards’ syndrome, sensitivity was similarly high, though slightly more false positives occurred. For Patau’s syndrome, detection was around 84%. Despite its accuracy, NIPT is still a screening test. A positive result from NIPT carries a positive predictive value of roughly 86% for Down syndrome in low-risk pregnancies, meaning about 14 out of 100 positive results turn out to be false alarms. Confirmation through amniocentesis or CVS is still recommended before making any decisions based on results.
The 20-Week Anomaly Scan
Around 18 to 22 weeks, most pregnant people are offered a detailed ultrasound that checks the baby’s physical development. This scan is thorough. The sonographer systematically examines the brain, skull, and face (including the lips, jaw, and eye sockets), the heart’s four chambers and major blood vessels, the lungs and chest, the spine in multiple views, the stomach and kidneys, the bladder, the umbilical cord insertion, and the long bones of the arms and legs, right down to the hands and feet. The placenta’s position and the cervix are also assessed.
This scan can detect a wide range of structural problems, from heart defects and cleft lip to spinal abnormalities and kidney issues. Some findings are immediately significant, while others require monitoring as the pregnancy progresses. Not all conditions are visible on ultrasound, so a normal scan is reassuring but not a guarantee.
Blood Type and Rhesus Factor
Early in pregnancy, a blood test determines your blood type and Rh status. This matters because if you are Rh-negative and your baby is Rh-positive, your immune system could produce antibodies that attack the baby’s red blood cells. This can lead to a serious condition called hemolytic disease of the fetus and newborn, where the baby’s red blood cells are destroyed faster than they can be replaced.
The risk is largely preventable. Rh-negative women who don’t already have these antibodies receive an injection of Rh immune globulin, typically around 28 weeks and again within 72 hours of delivery if the baby is Rh-positive. This injection prevents the immune system from becoming sensitized. Additional doses may be given after events like bleeding during pregnancy or miscarriage, since any mixing of maternal and fetal blood could trigger the immune response.
Infectious Disease Screening
At your first prenatal visit, blood tests screen for several infections that could affect the baby. In the United States, routine screening includes HIV, syphilis, and hepatitis B for all pregnant people, regardless of perceived risk or previous testing. Hepatitis C screening is also recommended during each pregnancy. Depending on your age and risk factors, testing for chlamydia and gonorrhea may be included as well.
Early detection is the point. Many of these infections can be treated or managed during pregnancy to dramatically reduce the chance of passing them to the baby. HIV treatment during pregnancy, for example, can lower transmission risk to under 1%. Syphilis left untreated can cause severe complications, but antibiotics given early enough can prevent harm to the baby entirely.
Gestational Diabetes Screening
After 24 weeks, screening for gestational diabetes is offered universally. This condition, where blood sugar levels become abnormally high during pregnancy, affects how the baby grows and can cause complications during delivery if unmanaged.
The screening approach varies by region. One common method is a two-step process: first a glucose challenge test where you drink a sugary solution and have your blood drawn an hour later, and if that result is elevated, a longer glucose tolerance test. The other approach uses a single 75-gram glucose tolerance test with blood drawn fasting, at one hour, and at two hours. Gestational diabetes is diagnosed when blood sugar exceeds specific thresholds at any of those time points: 92 mg/dL fasting, 180 mg/dL at one hour, or 153 mg/dL at two hours under the single-test method.
Most cases of gestational diabetes are managed through dietary changes and monitoring, though some people need medication. The condition typically resolves after delivery, but it does increase your long-term risk of developing type 2 diabetes later in life.
What Your Results Mean in Practice
A low-risk screening result is reassuring but not absolute. Screening catches the majority of affected pregnancies, but some conditions slip through with a normal result. A high-risk result, on the other hand, is not a diagnosis. It’s an invitation to discuss next steps, which usually means either more detailed screening or a diagnostic procedure.
The decision about which tests to have is yours. Some people want every available piece of information, while others prefer only certain tests. Your choices may depend on what you would do with the results, your comfort with the small procedural risks of diagnostic testing, and how much uncertainty you’re willing to sit with. There’s no single right approach, and the screening pathway can be tailored to what matters most to you.