The Third Trimester: Which Test Could Save Your Baby?

Written by the Healthline Editorial Team | Published on March 15, 2012
Medically Reviewed by Joan K. Lingen, MD

Prenatal Visits in the Third Trimester

During the third trimester you will start seeing your health care provider more frequently. Be sure to tell your doctor about any new developments and your questions or concerns at each visit. It is better to have all your questions answered during your visit than to have to call your doctor in the middle of the night with a question.

During office visits, you provide a urine sample, are weighed, have your blood pressure taken, and are examined by your doctor. During the physical exam, your doctor listens to the fetal heart rate, feels your arms and legs for edema (swelling), and measures your fundal height (the approximate the length of your uterus).

Determining the Baby's Position

In second half of the third trimester, your doctor tries to determine your baby's position. This is done by performing what is called Leopold's maneuvers - feeling the fetus by palpating first the fundus (top) of the uterus in the upper abdomen, then on either side of the uterus, and finally just above the pubic symphysis of the pelvis. Your doctor first tries to identify the head, which can be felt as a hard, round object. Opposite the head will be the buttocks, which will feel large and squishy. Then, your doctor feels for a long and regular spine. The hands and feet are opposite the spine. Ultrasound can help determine the baby's position, if Leopold's maneuvers are not sufficient.

It is important to identify fetuses that are breech (buttocks first) or transverse (sideways) before labor starts because these positions make delivery more complicated. If you know your baby is in a breech or a transverse position and you start labor or break your bag of waters, go to the hospital immediately.

Checking the Cervix

As you approach term, your provider will check your cervix. This provides information about how your body is preparing for birth. In a first pregnancy, it is common for the cervix to thin and soften (efface) before it actually opens (dilates). If you have had children before, it is not uncommon for your cervix to be dilated before softening or thinning occurs. The cervical examination helps determine whether or not you are really in labor if you arrive at the hospital with your cervix dilated less than 4 cm. Your doctor cannot determine from your cervical examination when you will start labor. If your doctor tells you your labor will start within a certain time, he/she is either guessing or is planning to induce labor (start your labor) if you do not start labor during that time. The cervical exam is also important for determining a plan of care if you are one of the 10% of women who do not deliver by their due date. Many doctors induce labor if the cervix is favorable (dilated and effaced) after 40 weeks gestation. If the cervix is unfavorable, your doctor will probably not start your labor and will give your cervix time to "ripen" and become more favorable.

Estimating the Weight of the Fetus

Your provider estimates the weight of the fetus. Conditions that can make estimating fetal weight less accurate include obesity, fibroids, and excess fluid. Ultrasound can be used to determine fetal weight, but it is not much more accurate than clinical estimates.

Large Fetus

Even if your doctor determines that your baby may be larger than normal, he will probably not recommend a cesarean section. Your body and pelvis are prepared for labor, and most women should attempt vaginal delivery. If the fetus is too large, labor generally stalls. Your provider may then decide to deliver your baby by cesarean section.

Some providers choose to induce labor in women they suspect have a large fetus (large for gestational age, or LGA). LGA pregnancies are often induced before the baby can become macrosomic (more than 4000-4500 grams) because of the risk for birth trauma and failure to progress in labor. (Failure to progress in labor occurs because the baby's head is unable to fit through the mother's pelvis.) Inducing labor is probably beneficial if the cervix is favorable. If not, it may be prudent to allow labor to start on its own. This maximizes the chances of a successful delivery.

Small Fetus

If your fetus is smaller than expected (small for gestational age, or SGA), your provider may order an ultrasound. If the fetus is very small at term, your doctor may want to induce your labor. There is usually no indication to speed up delivery if the baby has been consistently small throughout pregnancy. But if the baby has fallen off the growth curve near term, inducing labor may be indicated. At other times your provider may simply follow your progress with additional testing, such as a nonstress test (NST), oxytocin challenge test (OCT), or biophysical profile (BPP). If these tests are not reassuring, your doctor may go ahead and deliver the baby.

Assessment at the End of the Physical

After completing your physical exam, your provider should have answers to the following questions:

  • Is there appropriate fetal growth? At 35 weeks, the fundal height may no longer match the gestational age. (During the second trimester, the fundal height (length of your uterus) in centimeters is approximately equal to the age of the fetus in weeks. For example, if your fundal height is 26 cm, the fetus is about 26 weeks old.) This is partially due to the fetus settling into your pelvis, or "lightening." If your fundal height has grown appropriately and your doctor estimates that your fetus will likely weigh more than six pounds, your fetus is considered appropriately grown. If there is any question, your provider will order an ultrasound to assess the fetal weight and the amount of amniotic fluid.
  • Is your weight gain appropriate? As in the second trimester, your physician will tabulate how much weight you have gained over the entire pregnancy. In the third trimester, women generally gain one pound per week. If you have gained an excessive amount, your diet should be evaluated. If you are gaining weight much more rapidly than expected, you may be developing preeclampsia and your provider will want to evaluate you more frequently. If you are losing weight, you need to evaluate what you are doing. Some women find it very hard to eat in the third trimester because they get full so much earlier. If this is the case, try to eat more frequently. It is never good to lose weight during your pregnancy, even if you are overweight.
  • Is your blood pressure normal? Almost 10% of pregnancies are complicated by hypertensive disorders of pregnancy. It is normal for your blood pressure to rise in the third trimester. In many women it may approach or exceed your pre-pregnancy blood pressure measurements. However, an increase to 140/90, or, according to some authorities, an increase of 30 systolic (top number) or 15 diastolic (bottom number), may be a sign of preeclampsia and should be taken very seriously.
  • Do you have proteinuria? Protein in the urine (proteinuria) can be a sign of preeclampsia or impending problems. New onset of significant proteinuria (2+ or greater) requires a thorough evaluation for preeclampsia. If you have proteinuria and your blood pressure is normal, your provider will re-evaluate your blood pressure and fetal well-being again after 24 hours.

Testing During the Third Trimester

Different screening tests are used during the third trimester to check that your pregnancy is proceeding without complication and to detect any conditions that should be treated before delivery.

Rh Antibody Screen and RhoGAM if Rh-Negative

If you are Rh-negative and your partner is Rh-positive then you will need an Rh antibody screen at 28 weeks. The Rh antibody screen tests for Big D or the Rh antigen. If you have antibodies to Big D, then you have been exposed to Rh-positive blood and are now sensitized. This is potentially life threatening for the fetus. Your provider will outline a plan of care for you, and possibly have you see a maternal-fetal medicine specialist. If the Rh antibody screen is negative, then you should receive RhoGAM (immune glubulin). RhoGAM is simply an antibody to Rh, an anti-D immunoglobulin, and protects you from being exposed to Rh-positive cells and becoming sensitized. RhoGAM is effective for at least six weeks. If there's a possibility that you have been sensitized any time during your pregnancy-for example, if you have an amniocentesis, a significant abdominal trauma, vaginal bleeding, or an abruption (separation of the placenta) before delivery, you will be evaluated again and receive RhoGAM. Tell your doctor if you have any bleeding.

Repeat Hematocrit

Many providers test your hematocrit or blood count again in the third trimester. Your dose of iron (ferrous sulfate) can be increased if necessary to correct any anemia, prior to delivery. This test is not always repeated; if you have any concerns about it, talk to your provider.

Repeat Testing for STDs in High-Risk Populations

If you have previously had a sexually transmitted disease or are at a higher risk for contracting a sexually transmitted disease, your provider may check you for infection during the third trimester. Treating you for the infection can prevent your baby from being infected.

Group B Streptococcus

Group B Streptococcus is a neonatal infection that can cause brain damage, meningitis, and death. As many as 20% of women have this bacteria in their vaginas. These women have no symptoms that suggest an infection.

Two strategies are commonly used to prevent Group B streptococcus infection in the newborn. Some providers culture (grow in the laboratory) a sample from the vagina and rectum of their patients around 36 weeks; women who have group B streptococcus are give antibiotics during labor. Other providers treat all women who are at risk of infection during labor to prevent infection. Both approaches have pros and cons. Talk with your provider about which he/she will do.

Third Trimester Ultrasound

Ultrasound is useful in the third trimester to assess fetal growth and fetal anatomy. Ultrasound is not as accurate in determining gestational age.

Fetal Growth

Ultrasound in the third trimester is usually done to assess fetal growth. Poor weight gain, uterine size smaller or larger than expected for gestational age, or medical conditions such as hypertension may increase concern that the fetus' growth is abnormal. As in the second trimester, measurements of the fetal head, abdomen, and femur (thigh bone) are used to assess overall fetal size from the ultrasound image. If a previous examination is available for comparison, adequacy of fetal growth between the two ultrasounds can be assessed.

Fetal Anatomy

Fetal anatomy also may be assessed in the third trimester. Particularly important is evaluation of the organs that develop over time and may not manifest abnormalities until the third trimester. It is often useful to re-evaluate abnormal fetal anatomy seen during the second trimester in the third trimester to determine if the abnormality is stable or worsening with time. Follow-up examination may help determine prognosis and may help you make decisions about delivery. The decision-making process about the location for delivery (home birth, hospital), route of delivery (vaginal versus cesarean section), and who should be present at the time of delivery can often be helped by follow-up ultrasound assessment of a known abnormality. For example, a fetus with a worsening hydrocephalus (water on the brain) may be better delivered in a hospital with a neonatologist present and a pediatric neurosurgeon available.

Gestational Age

Determining gestational age by ultrasound in the third trimester is considerably less accurate than in first or second trimesters and is often wrong by plus or minus three weeks. The inaccuracy of determining gestational age in the third trimester is due to both the tremendous biological variability in fetal size in the third trimester and to the many factors that influence fetal growth. Fetuses of large parents tend to be genetically destined to be bigger in the third trimester compared to fetuses of relatively smaller parents. Factors affecting fetal growth may make the fetus large or small for actual gestational age.

An example of a medical condition influencing fetal growth is gestational diabetes. Diabetes due to pregnancy may result in a fetus that is extremely large at an early gestational age. If the mother has diabetes, a 32-week fetus may be as big as a normal, full-term baby. In this case, using ultrasound in the third trimester as the sole determinant of gestational age would overestimate the actual gestational age of the fetus. If delivered, this preterm fetus, although the size of a term fetus, would suffer the complications of prematurity. Another example of a medical condition that can affect fetal size is hypertension. If the mother has hypertension, a fetus in the third trimester may have difficulty thriving and consequently may be a small for gestational age. Ultrasound to determine gestational age would likely underestimate the gestational age. Doppler studies can evaluate the blood flow in the umbilical cord, fetal brain, or maternal uterine arteries and further help distinguish between a fetus that is small because of difficulty thriving in the intrauterine environment and a fetus that is small simply because of early gestation but is otherwise appropriately grown.

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