Second Trimester Tests: What to Know

The second trimester runs from week 14 through the end of week 27. It's often a busy stretch for prenatal care, with tests you'll likely have, like the anatomy scan and glucose screening, and others you can choose to have done, like genetic testing.

The names alone can be confusing: quad screen, NIPT, amnio, PUBS. You don't need to become an expert in every test. Knowing what each one is for helps you ask better questions and make choices that fit your values and your pregnancy.

Which second trimester tests are routine and which are optional?

• Routine for most people: the anatomy scan, glucose screening and Rh immune globulin if you're Rh-negative
• Optional: the quad screen, cell-free DNA screening (NIPT), amniocentesis and, in rare cases, PUBS
• Only if needed: a 3D ultrasound, when your provider wants a closer look at something

If you're looking for tests from earlier in pregnancy, like your first visit blood work or CVS, see first trimester tests.

Screening tests vs. diagnostic tests

Screening tests estimate the chance that your baby has a certain condition. They're low-risk, usually just a blood draw or an ultrasound, but they can't give you a definite answer. A "high-chance" result doesn't mean your baby has a condition. It means more testing may be offered.

Diagnostic tests give a more definite answer. Some, like amniocentesis, carry a small risk of miscarriage.

Second trimester ultrasounds

Fetal anatomy scan

When: 18–22 weeks

The anatomy scan, often called the 20-week ultrasound, is a detailed look at your baby from head to toe. It's usually done on your belly and takes 30 minutes or more. For many families, it's one of the most exciting appointments of pregnancy.

What it checks for: Your baby's brain, heart, spine, organs, limbs and growth. It also checks where the placenta is and how much amniotic fluid there is. Your provider may also measure the length of your cervix, which is sometimes done with a transvaginal ultrasound.

Good to know: If you'd like to learn your baby's sex, you can ask. If you'd rather be surprised, tell the sonographer before the scan starts. Sometimes your baby's position makes it hard to see everything, and you may be asked to come back for another look. That's common and doesn't mean something is wrong.

3D ultrasound — only if needed

When: Only if your provider needs a closer look

A 3D ultrasound builds a three-dimensional picture from ultrasound images. It isn't a routine part of prenatal care.

What it checks for: It can give a closer look at something seen on a regular ultrasound, like a possible cleft lip or a difference in the face or spine.

Good to know: The FDA advises against "keepsake" 3D or 4D ultrasounds done only for photos or videos. Medical ultrasounds are done by trained professionals, for a specific reason, and only as long as needed.

Routine blood tests

Glucose screening

When: 24–28 weeks

You'll drink a sweet glucose drink, then have your blood sugar checked about an hour later. You usually don't need to fast for this test. Your provider may recheck your blood count for anemia from the same blood draw.

What it checks for: Gestational diabetes. Pregnancy hormones make your body more resistant to insulin, and sometimes your body can't make enough extra insulin to keep up.

Good to know: A high result usually leads to a longer follow-up test, which does require fasting, not an automatic diagnosis. Some practices skip the one-hour test and use a single two-hour test instead. If you have concerns about the drink, ask whether alternatives are available.

Rh factor and Rh immune globulin

When: Your Rh factor is checked at your first prenatal visit. If you're Rh-negative, you'll usually get a shot around 28 weeks.

Most people are Rh-positive. If you're Rh-negative and your baby is Rh-positive, your body could make antibodies against your baby's blood. This usually doesn't affect a first pregnancy, but it can affect a future one.

What it protects against: A shot of Rh immune globulin, often called RhoGAM, keeps your body from making those antibodies.

Good to know: You may get another dose within 72 hours after birth if your baby is Rh-positive. You may also need one after bleeding, amniocentesis, CVS or an injury to your belly. If you're not sure whether you're Rh-negative, ask your provider.

Genetic screening

Every test in this section is optional. Your provider should offer them, but you decide whether to have any of them done.

Quad screen (multiple marker screening) — optional

When: 15–22 weeks, and most accurate between 16 and 18 weeks

You might hear this called the quad screen, the triple screen or multiple marker screening. They're all versions of the same blood test. It measures substances made by your baby and the placenta: alpha-fetoprotein (AFP), hCG, estriol and, in the quad screen, inhibin A. Today, the quad screen is the most common version.

What it checks for: The chance of Down syndrome, trisomy 18 and neural tube defects like spina bifida.

Good to know: A high-chance result doesn't mean your baby has a condition. Results can be thrown off if your due date is off, which is one reason an accurate early ultrasound matters. If your result is high-chance, your provider may offer a follow-up ultrasound, cell-free DNA screening, amniocentesis or genetic counseling.

Cell-free DNA screening (NIPT) — optional

When: Any time from 10 weeks

Cell-free DNA screening, also called noninvasive prenatal testing (NIPT), is a blood test. It looks at small pieces of DNA from the placenta that circulate in your bloodstream. Many people have it in the first trimester, but it can be done any time after 10 weeks.

What it checks for: The chance of Down syndrome and other chromosome conditions. It can also show your baby's sex, so let your provider know if you'd rather not find out.

Good to know: Cell-free DNA is more accurate than the quad screen, but it's still a screen, not a diagnosis. It doesn't screen for neural tube defects, so some people also have a blood test for AFP or rely on the anatomy scan for that.

Diagnostic tests

Diagnostic tests are usually offered after a high-chance screening result, or when there's another reason to look more closely, like a family history or something seen on ultrasound. Anyone can choose them, with or without screening first, and they're always optional.

Amniocentesis — optional

When: Usually 15–20 weeks

During an amniocentesis, or "amnio," a thin needle is guided by ultrasound through your belly to take a small sample of amniotic fluid. The fluid contains cells from your baby that can be tested.

What it checks for: Chromosome conditions like Down syndrome, some genetic conditions and neural tube defects.

Good to know: Amniocentesis gives a more definite answer than screening. When it's done by an experienced provider, the risk of miscarriage is about 1 to 3 in 1,000. You may feel some cramping or pressure during the procedure.

Percutaneous umbilical blood sampling (PUBS) — optional

When: After about 17–18 weeks, only if needed

PUBS, also called cordocentesis, takes a small sample of your baby's blood directly from the umbilical cord, using ultrasound to guide the needle.

What it checks for: Fetal anemia, blood or platelet problems, and infection. It's sometimes used when other tests couldn't give a clear answer.

Good to know: PUBS is rarely used today because safer options are usually available. The risk of miscarriage is higher than with amniocentesis, about 1 to 2 in 100.

Questions to ask before any prenatal test

You deserve to understand why a test is being offered and what it can tell you. Before you say yes or no, you can ask:

• Is this a screening test or a diagnostic test?
• What will the results tell us, and what won't they?
• What are the risks?
• When and how will I get my results?
• What happens if the result is abnormal?
• Would the results change my care or my baby's care?
• Is it covered by my insurance?

The BirthX takeaway

Prenatal tests can give you valuable information, but each one has a different purpose. Some are routine parts of care, and genetic tests are always your choice.

Knowing whether a test is a screen or a diagnosis, what it checks for, and what happens next can help you make choices that feel right for you. Download the one-page summary to bring to your next appointment. Whatever you decide, you deserve clear answers and care that respects your choices.

This article is for education only and is not a substitute for medical advice. Always talk with your provider about which tests are right for you.

Sources: MedlinePlus, Quadruple screen test; Mayo Clinic, Amniocentesis; American Pregnancy Association, Cordocentesis; KidsHealth, Prenatal test: PUBS; FDA, Avoid fetal "keepsake" images and heartbeat monitors; Loyola University Chicago Stritch School of Medicine, Prevention of RhD alloimmunization.