What Tests Are Done in Third Trimester of Pregnancy: Your Essential Guide to the Final Countdown

The final stretch of pregnancy is a whirlwind of anticipation, preparation, and a unique blend of excitement and nerves. As your due date draws near, your prenatal appointments become more frequent, and a new series of screenings and tests enters the picture. Understanding what tests are done in the third trimester of pregnancy is key to feeling empowered and informed, transforming medical jargon into a clear roadmap for your baby's final weeks of development. This isn't about finding problems; it's about proactive wellness checks, ensuring both you and your little one are perfectly poised for a healthy birth.

The Foundation: Routine Check-ups and Their Importance

Before diving into specific diagnostic tests, it's crucial to understand the bedrock of third-trimester care: your routine prenatal visits. From around week 28 until delivery, these appointments typically shift from every four weeks to every two weeks, and then to weekly starting at week 36. While they may feel repetitive, each visit is a vital snapshot of your and your baby's well-being.

At each of these visits, you can expect a standard set of checks that form the baseline for your care:

Weight and Blood Pressure Monitoring: Steady weight gain is expected, but a sudden spike can sometimes indicate preeclampsia, a condition characterized by high blood pressure. Your blood pressure is checked meticulously at every visit for this very reason.
Urine Analysis: A simple dipstick test of your urine checks for protein (a potential sign of preeclampsia) and sugar (which could indicate gestational diabetes).
Fundal Height Measurement: Your healthcare provider will measure the distance from your pubic bone to the top of your uterus (the fundus) with a tape measure. This measurement in centimeters should roughly correlate with your baby's gestational age in weeks. A significant deviation might prompt further investigation into baby's growth.
Fetal Heart Rate Check: Using a handheld Doppler device, your provider will listen to your baby's heartbeat at every appointment to ensure it's within a healthy, normal range.
Fetal Position Check: As you progress through the trimester, your provider will feel your abdomen (a process called Leopold's maneuvers) to determine the baby's position—whether they are head-down (cephalic), breech, or transverse.

Screening for Common Conditions: The Group B Streptococcus (GBS) Test

One of the most universal third-trimester tests is the Group B Streptococcus (GBS) screening. Group B Strep is a common type of bacteria that often lives harmlessly in the intestinal tract and, in about 25% of all healthy women, in the vagina or rectum. While it's generally harmless to healthy adults, it can be passed to the baby during a vaginal delivery and cause serious, even life-threatening infections in newborns, such as pneumonia, meningitis, or sepsis.

The test is typically performed between weeks 36 and 38 of pregnancy. The procedure is straightforward and painless: your healthcare provider will use a cotton swab to take samples from your vagina and rectum. The swab is then sent to a lab for culture.

If you test positive for GBS, it does not mean you are unclean or sick; it simply means you are a carrier. The treatment is simple and highly effective: during labor, you will be given intravenous (IV) antibiotics. This treatment dramatically reduces the risk of transmitting the bacteria to your newborn. Penicillin is the most common antibiotic used, but an alternative will be prescribed if you have an allergy. This is a prime example of a third-trimester test that is purely preventative and a major success story in modern obstetrics.

Assessing Fetal Well-being: The Biophysical Profile (BPP)

If there are any concerns about your baby's health or growth, or if you have a high-risk pregnancy (due to conditions like hypertension, diabetes, or going past your due date), your provider may order a Biophysical Profile (BPP). This is a non-invasive, combined test that uses ultrasound and a non-stress test to evaluate your baby's well-being in a comprehensive way. It checks five specific areas, each scoring 0 or 2 points, for a total possible score of 10.

The five components of a BPP are:

Non-Stress Test (NST): Though often performed separately, it's integrated into the BPP. You'll recline in a chair with two sensors placed on your abdomen—one to monitor the baby's heart rate and another to measure contractions. You'll hold a button to press every time you feel the baby move. The test looks for accelerations in the fetal heart rate in response to movement, which is a sign of a healthy, well-oxygenated nervous system. This part typically takes 20-40 minutes.
Fetal Breathing Movements: The ultrasound technician watches for at least 30 seconds of sustained rhythmic breathing movements by the baby within a 30-minute observation period. This practice is a good sign of lung maturity and health.
Fetal Movement: The technician looks for at least three discrete body or limb movements within the 30-minute period. This indicates normal neurological function.
Fetal Tone: The ultrasound assesses for at least one episode of a limb moving from a flexed position to an extended position and then returning to flexed. Good muscle tone is a positive sign.
Amniotic Fluid Volume: The sonographer measures the deepest pocket of amniotic fluid in your uterus. An adequate amount of fluid is crucial for cushioning the baby and allowing for movement and growth. Low fluid (oligohydramnios) can be a concern.

A score of 8-10 is generally considered reassuring. A score of 6 is considered equivocal and usually means the test will be repeated within 24 hours. A score of 4 or less is cause for concern and may indicate the need for further evaluation or immediate delivery, depending on the circumstances.

Tracking Baby's Growth: The Ultrasound for Fetal Growth

While a detailed anatomy scan is typically performed in the second trimester, you may have another ultrasound in the third trimester. This is not routine for every pregnancy but is ordered if your fundal height measurement is off, if you have a medical condition like diabetes, or if there are concerns about the baby's size (being too small or too large).

This growth ultrasound estimates the baby's weight by taking several key measurements:

Head Circumference (HC)
Abdominal Circumference (AC)
Femur Length (FL)

These measurements are plotted on a growth chart to estimate the baby's weight percentile. The AC is particularly important as it reflects the baby's nutritional state. The ultrasound also provides another opportunity to check amniotic fluid levels and confirm the baby's position.

The Non-Stress Test (NST) as a Standalone Procedure

As mentioned, the Non-Stress Test is a key part of the BPP, but it is also one of the most common third-trimester tests ordered on its own. It is a primary tool for monitoring babies in high-risk pregnancies. The goal is simple: to confirm that the baby is receiving enough oxygen and that their nervous system is responding appropriately.

The test is termed "non-stress" because it places no stress on the baby and is completely non-invasive for the mother. A "reactive" result—showing two or more heart rate accelerations over a 20-minute period—is considered normal and reassuring. A "non-reactive" result doesn't automatically mean something is wrong; the baby might just be asleep. In such cases, the test might be extended, or a small noise-making device might be buzzed near the abdomen to wake the baby. If the test remains non-reactive, it will usually be followed by a BPP for a more complete assessment.

Preparing for Labor: The Cervical Check

As you approach your due date, particularly after week 36 or 37, your healthcare provider may offer to perform a cervical check during your weekly appointments. This is a manual physical exam to assess whether your body is beginning to prepare for labor.

During the check, your provider will insert a gloved finger into your cervix to assess three things:

Dilation: How open your cervix is, measured in centimeters from 0 (closed) to 10 (fully dilated).
Effacement: How thin your cervix is, expressed as a percentage from 0% (thick) to 100% (paper-thin).
Station: How low the baby's head is in the birth canal, measured on a scale of -5 (high) to +5 (crowning).

It is critical to know that this exam is entirely optional. The information it provides can be unpredictable. You could be 3 cm dilated for weeks, or you could be closed tight and go into labor that night. For some, knowing the progress is encouraging; for others, it can lead to anxiety and disappointment. You have every right to decline these checks until you are in active labor if you prefer.

Specialized and Follow-up Blood Work

Depending on your health history and earlier test results, you may have some follow-up blood tests in the third trimester.

Repeat Hematocrit/Hemoglobin: Your blood count is often rechecked to screen for anemia, which is common in late pregnancy due to increased blood volume.
Antibody Titer (if Rh-negative): If you have Rh-negative blood, you likely received a Rho(D) immune globulin injection around week 28 to prevent your body from developing antibodies that could attack a baby with Rh-positive blood. A follow-up blood test may be done to confirm no antibodies have developed.
Glucose Tolerance Test (if indicated): If you failed the one-hour glucose test in the second trimester but passed the longer three-hour test, your provider might recommend repeating the one-hour test in the third trimester to ensure gestational diabetes has not developed later on.
Testing for Sexually Transmitted Infections (STIs): In some cases, particularly if there is a new concern or risk, your provider may recommend repeat testing for infections like syphilis, HIV, or hepatitis B to ensure you remain negative before delivery.

Every kick, every flutter, every test, and every check-up in these final weeks is a step on the path to meeting your baby. The battery of third-trimester tests is not designed to worry you but to equip your medical team with the deepest possible understanding of your unique pregnancy, allowing them to safeguard your health and your baby's with precision and care. This knowledge is your power, turning the unknown into a managed journey and giving you the confidence to focus on what truly matters—the incredible moment when you finally get to hold your healthy newborn in your arms.

Zurück zum Blog