You’re in the final stretch! Not only are you almost ready to meet your baby, but you can rest assured that testing during the third trimester is a lot less overwhelming than your first trimester tests or your second trimester tests. However, you’ll still have some testing decisions to make before you’re snuggling your sweet bambino.


What it is?
Just your regular old ultrasound—a peek at the baby.

It’s more the reasons for the ultrasound that are important. Third trimester ultrasound screening is offered for a variety of reasons: to check the size and position of the baby, to determine amniotic fluid levels, to check the position of the placenta, and to monitor the baby’s movements.

How is it done?
This ultrasound isn’t any different than all the others, except you may notice there’s a lot less of your baby on the screen now that he or she has gotten so big!

How to prepare?
Third trimester ultrasounds aren’t routine. By now, you should be aware of any problems with your baby’s genes or development.

Sometimes third trimester ultrasounds may lead to unwanted and unnecessary interventions, especially if you feel like there isn’t a good reason they are being done.

If it seems like your provider is pushing an ultrasound, ask him or her what the purpose of the ultrasound is. For example, if they want to make sure the baby isn’t “too big,” you could choose to decline this ultrasound knowing that your body will grow exactly the right size baby for you to birth.

Remember, ultrasound measurements of your baby’s weight can be off by up to two pounds!

If it seems like the provider has a good reason to advise an ultrasound, remember that you can negotiate how long and detailed that ultrasound is. For example, if your doctor or midwife wants to ensure the baby is head down, perhaps your provider can take a quick peek themselves with an ultrasound machine rather than sending you for a lengthier scan.

Here is more information on the benefits vs. risks of baby ultrasounds.

Assuming you’re healthy and low-risk and haven’t had any problems with the pregnancy, there’s rarely a reason for a third trimester ultrasound.

Group B Strep (GBS) Testing (~36 weeks)

What it is?
Group B Strep testing is a swab of the vagina and anus to test for a bacteria called Group B Strep. This bacteria is normal flora in about 25% of women, but can cause an infection in the baby particularly if the baby is preterm or the mother’s bag of water has been broken for a long time prior to the birth.

In the United States, women who test positive for GBS are treated with antibiotics during labor to prevent the baby from becoming infected with GBS.

How is it done?
Your doctor or midwife will swab your vagina and anus and send the sample to the lab to check for Group B Strep. Your results should be available by the next time you return for an appointment.

On occasion, you may test positive for GBS in your urine at some point during the pregnancy. If GBS is present in the urine, you are considered a “heavy colonizer” and there is a higher risk that your baby will become colonized (but not necessarily infected) with GBS.

If you are a heavy colonizer, you won’t have the screening test at 36 weeks—once you test positive in the urine, you are considered GBS positive for the remainder of the pregnancy.

How to prepare?
There’s not much you need to do to prepare for the test itself (it’s quick and painless), but you can do footwork early in your pregnancy. By eating a low sugar diet and consuming probiotics or cultured foods like yogurt, you may be able to keep your bacteria levels in check. If you still test positive, you do have some options about the testing as well as how the results will be handled.

Although the United States tests all women for GBS, many other countries take a “risk based approach” in which advance testing is not done and women only get antibiotics if risk factors are present. Some such risk factors include:

  • GBS in the urine at some point during the current pregnancy
  • previously gave birth to an infant with an early GBS infection
  • goes into labor at less than 37 weeks
  • gets a fever during labor
  • water broken for greater than 18 hours

You could consider asking your provider about using the risk based approach if you are concerned about receiving antibiotics. There has not been much research about the effects of antibiotics given during labor on the mother’s or baby’s microbiome.

There are also some other alternatives to antibiotics, including douching with Hibiclens during labor.

You also could take action to try to prevent GBS before it starts with probiotics.

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Non-Stress Test (NST)

What it is?
A non-stress test is a monitoring test that checks on your baby’s healthy by monitoring the baby’s heart rate for a 20-30 minute period.

For women who are low-risk, the most common reason for a non-stress test is because you are past your due date. Many providers will recommend NST’s 1-2 times per week after 40 or 41 weeks of pregnancy.

What your doctor or midwife wants to see on the non-stress test is that your baby’s heart rate has lots of variability (minor changes in the heart rate that are an indicator the baby is doing well), some accelerations (increases in the baby’s heart rate that also confirm well-being), and absence of decelerations (decreases in the baby’s heart rate that may indicate your baby is not faring well and delivery may be a good option).

Some women may start NSTs earlier in their pregnancy if they have specific risk factors such as an abnormality on ultrasound, are over age 35, or have diabetes or high blood pressure.

How is it done?
Two monitors are strapped to your abdomen—one that will detect contractions (frequency, not strength) and one that will detect the baby’s heartbeat.

Both monitors will create a tracing with the pattern of the baby’s heart rate and contractions so that your doctor or midwife can not only see the heart rate pattern but also see how the baby’s heart rate responds to contractions if you are having any.

Based on the heart rate reading, your provider will classify your baby as “reactive” or “non-reactive.” Basically, reactive means you passed the test, all is well with baby, and it’s reassuring that you can continue the pregnancy for at least several more days. Non-reactive means that your provider may want to use other tests to check on the baby’s well being, such as a biophysical profile.

How to prepare?
Because active babies tend to have more variability and accelerations in their heart rate (which is what we want to see!) it’s a good idea to eat and drink just prior to the procedure to perk your baby up a little bit.

If your baby is too sleepy during the procedure, it may take longer to get an accurate reading on the baby’s well-being.

Biophysical Profile

What it is?
A biophysical profile is another test offered during the third trimester to check on your baby’s well-being. Like the non-stress test, the most common reason this test is done in a low risk woman is because she is still pregnant past her due date.

A biophysical profile actually consists of two elements:

  1. Ultrasound
  2. Non-stress test (as described above)

Women may have biophysical profiles more routinely if they are pregnant with twins or triplets, had a previous stillbirth, have health problems, or have a baby who is anticipated to have a genetic or congenital abnormality at birth.

How is it done?
The non-stress test portion of the biophysical profile is done exactly like a non-stress test is done by itself. Your baby will receive a score for it’s heart rate depending if it is reactive or non-reactive.

The ultrasound portion of the biophysical profile involves examining and giving a score to the following aspects:

  • baby’s movements
  • baby’s breathing
  • baby’s muscle tone
  • amniotic fluid level (AFI)

The ultrasound can take anywhere from just a few minutes to 30 minutes depending how active your baby is.

Heart rate (from the NST), movement, breathing, muscle tone, and amniotic fluid level are all worth 2 points each for a total score of 10.

  • If everything looks great, you’ll get a perfect 10!
  • An 8 out of 10 is also a good score that reassures you and your provider that your baby is doing well.
  • Anything 6 or below may indicate that the test needs to be repeated or your baby may need to be born sooner rather than later, depending how far along you are in the pregnancy.

How to prepare?
Prepare for the non-stress test as described above—be sure that you eat and drink just prior to the procedure so that the baby is active.

The biophysical profile will be done immediately after the non-stress test, so you’ll benefit from the fact that you have eaten and drank something recently. It’s also important to ensure that you are drinking plenty of water in the 24 hours prior to the test since dehydration may make amniotic fluid levels low, which can be a reason for induction of labor.

If amniotic fluid is the only measure that is low on the test, you may negotiate with your provider that you hydrate adequately (drink at least 2 L of fluid) and return 24 hours later for another check.

Third Trimester Lab Tests

What it is?
Although not quite as comprehensive as the blood work done during the first trimester, blood work done during the third trimester still provides plenty of important information. Lab tests include:

  • CBC: checks for anemia
  • RPR: checks for syphilis
  • GC/CT: checks for gonorrhea and chlamydia

How is it done?
Just a simple blood test! The test is usually done between 26 and 30 weeks around the same time as the gestational diabetes screen, so by some definitions these tests could actually occur at the end of the second trimester as well as the third trimester.

How to prepare?
There’s really not much you need to do to prepare. Keep in mind that some women decide to decline the RPR and GC/CT if they aren’t concerned about sexually transmitted infections and/or already tested negative for these during the first trimester.

Good luck with your third trimester tests!

Given all of the strain on your body and your brain in the third trimester as you make those final preparations for the birth of your baby, it’s a gift that there are fewer decisions to make about prenatal testing. You probably won’t even be offered an NST or BPP unless you have risk factors or go past your due date. If you have a more holistic provider, you may not even be offered a third trimester ultrasound either—that leaves you with plenty of time to focus on preparing for labor.