Prenatal testing in the second half of pregnancy is a different conversation than the testing offered earlier. Many of these tests produce results in real time, and many can directly affect your birth plan. As a result, decisions at this stage can feel heavier, both because you are closer to birth and because the consequences of these results land more immediately. This episode is a comprehensive, evidence-based guide to prenatal testing in the second half of pregnancy. We walk through gestational diabetes screening, group B strep testing, fetal well-being tests, and growth ultrasounds. We also cover what to do when test results prompt additional evaluation and what is unique about partner involvement when results arrive in real time.

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A Different Stage of Testing

This episode is a companion to the first half of the pregnancy guide on prenatal testing. The first episode covers all of the testing offered through your anatomy ultrasound. That includes bloodwork, carrier screening, genetic screening, the anatomy ultrasound, and diagnostic testing. If you have not listened to that episode yet, you can find it here.

Testing in the second half of pregnancy is different in a few important ways. First, many tests in this stage produce results in real time, while you are still in the appointment. The results can also directly affect your birth plan. For example, a result might determine whether you receive antibiotics during labor, whether your baby’s wellbeing warrants additional monitoring, or whether a glucose result requires diet or medication management. As a result, decisions in this stage can feel heavier, both because you are closer to birth and because the consequences of these results land more immediately.

A Quick Refresher

Here is a brief refresher on foundational principles you need to understand about prenatal tests. Prenatal tests fall into two main categories. Screening tests tell you the likelihood that a condition is present. Diagnostic tests give you a definitive answer. If you receive unfavorable results from a screening test, do not jump to the worst-case scenario. A screening test can only tell you that you may want to consider additional testing. It does not deliver a diagnosis.

Another important thing to keep in mind is that every prenatal test is optional and should include informed consent. That means your provider explains what the test is, why it is being offered, what the results can and cannot tell you, any risks, and any alternatives. You always have the right to decline any test or ask about your other options.

Gestational Diabetes Testing

During pregnancy, your body naturally becomes more resistant to insulin. As a result, more glucose stays in your blood. For most women, the pancreas compensates by producing additional insulin. However, for some, the pancreas cannot keep up, and blood glucose climbs. This creates a condition called gestational diabetes. It is common and manageable when identified. Nearly all pregnant women in the United States get this screening, even without risk factors.

Gestational diabetes affects roughly 8% of US pregnancies based on the most recent CDC data. The rate has been steadily climbing. A 2025 analysis of more than 12 million US births found that gestational diabetes increased 36% from 2016 to 2024.

Testing typically happens in two stages. The first stage is the glucose challenge screening, performed between 24 and 28 weeks. If you have risk factors for diabetes, including a history of gestational diabetes in a previous pregnancy, your provider may test you earlier. For the screening test, you do not need to fast. You drink a sweet glucose solution and have your blood drawn one hour later. A blood sugar level above a threshold (usually 130 to 140 milligrams per deciliter) indicates further testing is warranted. Most women who screen positive on the initial test are not diagnosed with gestational diabetes on follow-up testing.

If your screening test is positive, the next step is a diagnostic test. The diagnostic test is the three-hour glucose tolerance test. For this one, you do need to fast, typically for at least eight hours beforehand. When you arrive, your provider draws a baseline blood sample to measure your fasting blood sugar. Then you drink a larger glucose solution. You have your blood drawn again at one, two, and three hours after the drink. If two or more of your readings come back elevated, your provider will diagnose you with gestational diabetes.

You may have additional options if you know what those options are. Some providers offer alternatives to the glucose drink, like testing with a measured serving of jelly beans. Some providers will consider home glucose monitoring for a period of time as an alternative to the standard test. The glucose drink is very sweet and contains dyes and other ingredients that some expecting mothers wish to avoid. Not every provider offers alternatives. The key takeaway here is that you don’t know all your options unless you ask.

What Happens After a Gestational Diabetes Diagnosis

If you are diagnosed with gestational diabetes, the first line of management is typically dietary changes and blood glucose monitoring. Your provider may refer you to a dietitian or diabetes educator to help you learn how to balance your meals. The general approach focuses on consistent carbohydrate intake spread across smaller meals and snacks. Regular physical activity also helps your body use insulin more effectively. Walking after meals is one of the simplest and most effective interventions.

You will also begin checking your blood glucose at home. This typically means once in the morning when you wake up, and then following each meal. Home blood sugar testing uses a small lancet device to prick your fingertip. You place a drop of blood on a test strip and read the result on a glucose meter. The whole process takes a few seconds. It is mildly painful at first, but most women get used to it quickly and find it is not a big deal.

Your care provider will give you target ranges, and most expecting moms with gestational diabetes can manage their blood sugar through diet and exercise alone. However, if your numbers stay above target, your provider may recommend medication. The most common options are insulin, which has been the standard treatment for decades, and metformin, an oral medication that has become more widely used in recent years.

Gestational diabetes also changes how your pregnancy is monitored. You may have additional growth ultrasounds to track your baby’s size. Your provider may also recommend more frequent fetal well-being tests as you approach your due date. We will cover both of these later in the episode. For a more detailed walkthrough of testing and treatment, the Pregnancy Podcast has a full episode on navigating gestational diabetes from testing to treatment.

Group B Strep Testing

Group B strep, or GBS, is a common bacterium. It lives harmlessly in the gastrointestinal tract, vagina, and rectum of many healthy adults. According to the American College of Obstetricians and Gynecologists, the prevalence in pregnant women is between 10% and 30%. It is reported to be higher in black women and may vary by geographic location. The concern in pregnancy is that GBS can pass to your baby during birth. The vast majority of babies exposed to GBS during birth do not become sick. A small number develop infections.

The screening-and-treatment system in the United States has dramatically reduced infant GBS infections. According to ACOG, implementation of national guidelines for intrapartum antibiotic prophylaxis has reduced early-onset GBS infections by more than 80%. Specifically, infections declined from 1.8 per 1,000 live births in the 1990s to 0.23 per 1,000 live births in 2015.

Because GBS colonization can come and go, testing happens close to birth. ACOG recommends universal GBS screening between 36 and 37 weeks 6 days. The test is a simple swab of your vagina and rectum. Results are typically available within 24 to 48 hours. If you test positive, your provider will offer intravenous antibiotics during labor to reduce the chance of transmission to your baby.

Antibiotic Timing and Effectiveness

The effectiveness of antibiotics for preventing early-onset GBS depends significantly on timing. Research has shown that 4 or more hours of penicillin or ampicillin before delivery is highly effective. Effectiveness is around 91% in term babies and 86% in preterm babies. There is more to this, and if you test positive for GBS, I highly recommend you listen to this episode.

Generally, GBS does not transmit to a baby during a cesarean. The reason is that your baby does not pass through the birth canal, where GBS lives. However, GBS testing is still recommended even if you have a planned cesarean. It is possible that labor starts or your water breaks before the scheduled cesarean date, which increases the risk of transmission.

There is some interesting research on whether probiotics can reduce GBS colonization. A 2022 systematic review and meta-analysis found that antenatal probiotic use decreased the probability of a positive GBS result by 44%. Other studies have shown more mixed results. The evidence is encouraging but not conclusive. For a full deep dive on probiotics, antibiotic management, and treatment options if you test positive, the Pregnancy Podcast has a complete episode on group B strep.

Fetal Wellbeing Tests

Beyond gestational diabetes and group B strep, some tests in the second half of pregnancy focus specifically on assessing your baby’s wellbeing. Providers typically offer these if there is a specific concern. For example, decreased fetal movement, a high-risk pregnancy, or a pregnancy past the due date can prompt these tests. Concerns about placental function or fetal growth may also lead to fetal well-being testing. Conditions like gestational diabetes, high blood pressure, and preeclampsia often involve fetal well-being surveillance as part of routine care.

The Nonstress Test

A nonstress test, usually shortened to NST, monitors your baby’s heart rate and your uterine activity over 20 to 30 minutes. It is called nonstress because no stress is placed on your baby. It is just an observation. Your provider places two belts on your abdomen. One picks up the fetal heart rate, and the other tracks any contractions.

The test looks for accelerations, which are small temporary increases in your baby’s heart rate, typically in response to movement. A reactive NST means your baby’s heart rate accelerated in response to movement in the expected pattern. This is reassuring. A non-reactive NST means the expected accelerations did not occur. However, a non-reactive result does not necessarily mean something is wrong. For example, your baby may simply have been asleep during the test. If your test is non-reactive, your provider will typically follow up with additional evaluation.

Modified Biophysical Profile

There is also a modified biophysical profile that combines a nonstress test with a measurement of amniotic fluid volume. Your provider measures amniotic fluid volume via ultrasound, using either a single deepest pocket measurement or an amniotic fluid index calculation. Both are quick. The whole modified BPP typically takes about 20 to 25 minutes.

Full Biophysical Profile

Then there is a full biophysical profile, or BPP. This includes the nonstress test plus an extended ultrasound evaluation. It scores five aspects of your baby’s wellbeing. These are fetal breathing movements, body movements, muscle tone, heart rate reactivity (the NST component), and the volume of amniotic fluid around your baby. Each receives 0 or 2 points, for a possible total of 10. Higher scores are reassuring. Lower scores prompt further evaluation and, in some cases, consideration of early delivery. Full biophysical profiles are sometimes done at regular intervals, for example, weekly, in high-risk pregnancies.

Of these two options, the modified BPP is the most commonly used initial assessment when fetal well-being testing is needed. It is faster than a full BPP because it does not require an extended ultrasound observation period. The full BPP scores additional aspects of fetal well-being that need 30 minutes of observation to evaluate. Research supports the use of the modified BPP as an effective initial test in many situations, with performance comparable to the full BPP in the populations for which it is typically used. As a result, providers reserve the full BPP for situations where a modified BPP is non-reactive or there is a more specific concern.

What Happens After a Concerning Wellbeing Result

Walking out of an appointment with a non-reactive NST or a low biophysical profile score can be one of the more stressful moments in pregnancy. However, knowing what typically comes next can help. The first thing to know is that a single concerning result does not automatically mean something is wrong. In many cases, additional evaluation gives a clearer picture before any decisions are made.

After a non-reactive NST, the next step is often a modified or full biophysical profile. Providers sometimes use vibroacoustic stimulation, which is a brief sound stimulus, to wake your baby and re-evaluate. If a BPP score is low, your provider may extend the observation period, repeat the test, or order additional studies, such as a Doppler ultrasound, to assess blood flow through the umbilical cord.

If multiple tests continue to raise concerns, your provider will likely discuss delivery timing with you. The decision depends on many factors. Gestational age, the specific findings, your overall pregnancy history, and your baby’s growth all factor in. As a result, what looks like a worrying result in one situation may lead to repeat monitoring, while in another it may lead to a conversation about induction or cesarean. Your provider will walk you through what makes sense for your specific situation.

Growth Ultrasounds

Beyond fetal well-being tests, you may have additional ultrasounds in the second half of pregnancy. These are called growth scans. Providers use them to estimate your baby’s size and check the amount of amniotic fluid. They are not routine in low-risk pregnancies. However, they may be ordered for specific reasons.

One common trigger is a fundal height measurement that does not match your gestational age. Fundal height is the distance from your pubic bone to the top of your uterus, measured in centimeters. From about 24 to 38 weeks, the measurement should roughly match your number of weeks pregnant, give or take a few centimeters. If your measurements are consistently outside that range, your provider may order a growth scan to clarify.

Other reasons for growth scans include conditions like gestational diabetes, high blood pressure, or a prior pregnancy with growth concerns. Twin or multiple pregnancies typically involve growth scans throughout the second half of pregnancy. It is important to know that growth ultrasound estimates are not exact. One study found that ultrasounds were accurate within 10% of the actual birth weight about 84% of the time. Your provider will look at the overall pattern over time, not just one number, to decide whether anything needs further attention. For a deeper dive on the accuracy and limitations of ultrasound during pregnancy, the Pregnancy Podcast has a complete episode on ultrasound during pregnancy.

Decreased Fetal Movement

By the third trimester, you should have a sense of your baby’s typical movement patterns. You will know roughly when your baby is most active, what kinds of movements feel normal, and how often you feel them. Paying attention to your baby’s movement patterns is one way to stay connected to your pregnancy at this stage. If you notice a meaningful decrease in your baby’s movement, call your provider. Most of the time, decreased movement is not a sign of a problem. However, it is much better to go in for a quick check than to stress about something being wrong.

This topic genuinely deserves its own deep dive, and we will cover it in a future episode. The short version is to trust your instincts. Anytime a symptom comes up that concerns you, call your doctor or midwife. That is what they are there for. Even if it turns out to be a false alarm, getting that reassurance is much better than sitting at home worried.

Partner Involvement for Second-Half Testing

Ideally, your partner attends every prenatal appointment with you. While that can be a big time commitment, it is such a great opportunity for them to get involved in your pregnancy and to ask questions. Going through these appointments together makes the whole experience more shared.

Many of the tests in the second half of pregnancy produce results in real time. A nonstress test result is reactive or non-reactive while you are still in the appointment. A biophysical profile score is something you process together as the ultrasound is happening. Group B strep results take a few days to come back, but the conversation about what a positive result means and what happens during labor is one you should have together. It can be a huge help to have your partner or another support person present for these moments.

There is also something specific about late-pregnancy testing decisions. By this point, you are tired. You are physically uncomfortable. You are emotionally close to meeting your baby. As a result, decisions feel heavier. A test result that prompts a conversation about an induction, a recommendation to come back for additional testing, or a change to your birth plan is harder to absorb on your own. It is much easier to process with someone who knows you and can help you think through it. The goal is to have someone there to support you as you process what you are being told and to help you understand the next steps and your options.

The Big Picture

You don’t need to memorize every threshold or remember every detail about each test. Going into any of these appointments knowing what to expect and what comes next can ease some of the anxiety and help you prepare.

It can be stressful to have additional tests, especially as you get closer to birth. However, remember what all of this testing actually represents. This is your doctor or midwife monitoring you and your baby to make sure everything is okay. Every test is a tool that helps your care team catch anything that needs attention early. In that sense, all of this testing is a good thing, even if it feels stressful in the moment.

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