Pregnancy: Third Trimester (28 weeks to delivery)

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If a woman participates in high-risk activities that may transmit infections such as HIV or hepatitis B, such as unprotected sexual contact or intravenous drug use, retesting for these infections during the third trimester is recommended. It is also recommended, and in some states required, that testing for sexually transmitted diseases like gonorrhea, chlamydia, and syphilis be performed or repeated in the third trimester. (See the Pre-conception section for additional information.)

The pregnant woman may also be given a non-stress test (NST). This non-invasive test, given after week 28, is used mostly in high-risk pregnancies or when a baby is past due to monitor its health and look for signs of distress. The test measures the fetal heart rate as it relates to movement.

When the healthcare practitioner is concerned about how contractions will affect the fetal heart rate, the non-stress test may be followed by a contraction stress test. The woman is given a medication that induces mild contractions (stress), after which the heart rate of the fetus is again monitored.

Urine screen for protein and/or glucose

At each prenatal visit through the first, second, and third trimesters, the expectant mother may be asked to give a urine specimen, which will usually be tested in the office using a dipstick to screen for the presence of glucose (sugar) and/or protein. Although small amounts of both glucose and protein are normally present in urine, high levels can indicate a problem and may call for further testing.


A high level of protein in the urine is a warning sign. It may indicate kidney damage or disease or it may be a transient elevation due to, for example, an infection, medication, or emotional or physical stress. Examples of additional tests that may be done to help determine the cause include a 24-hour urine protein testfull urinalysis, or urine culture (to identify any bacteria or yeast present).

Of particular concern during the second and third trimesters is preeclampsia (sometimes called toxemia or pregnancy-induced hypertension), a disorder characterized by high blood pressure and large amounts of protein in the urine that occurs in approximately 8% of all pregnancies. Symptoms include swelling, sudden weight gain, headache, and vision changes. Risk factors include first pregnancy, being pregnant with more than one child, age (teenagers and women over 40 years of age), being African American, and having pre-existing diabetes, hypertension, or kidney disease. It can result in a decrease of air and nutrition getting to the baby through the placenta, causing low birth weight or other complications. If caught early enough, however, through routine checking of blood pressure and urine protein levels, health problems for the mother and baby due to pre-eclampsia can be managed.


High urine glucose levels may be a sign of undiagnosed diabetes that is already present in the mother or gestational diabetes, a form of diabetes that can develop during pregnancy. A positive urine test for glucose will usually be followed by a confirmatory blood glucose test, which is also routinely used to screen for gestational diabetes during the second trimester (between 24 and 28 weeks of pregnancy).

Urine culture to detect bacteria in the urine (may be repeated)

Several organizations, including the U.S. Preventive Services Task Force (USPSTF), the Infectious Diseases Society of America (IDSA), the American Academy of Family Physicians (AAFP), and the American College of Obstetricians and Gynecologists (ACOG), recommend that pregnant women be screened for asymptomatic bacteriuria using a urine culture between 12 and 16 weeks gestation or at the first prenatal visit, if later (ACOG recommends first prenatal visit and repeated in the third trimester).

Asymptomatic bacteriuria in pregnancy is diagnosed when significant amounts of bacteria are found by urine culture but the woman is not experiencing any symptoms associated with a urinary tract infection, such as pain or urgency to urinate. This condition occurs in approximately 2%-10% of pregnant women in the U.S. and can lead to more serious kidney infection as well as increases the risk of preterm delivery and low birth weight. Treatment with appropriate antibiotics is advised.

Group B strep screen

Group B streptococcus (GBS) are bacteria that are present as part of the normal flora in the vagina and gastrointestinal areas of about 25% of women. This is different than the more commonly known Group A streptococcus that causes strep throat.

GBS is usually not a problem, except when it is present in the vagina during delivery. In this case, it can spread to infect the uterus, amniotic fluid, urinary tract, and any incision made during a cesarean section. At delivery, when the baby passes through the birth canal, the bacteria can be inhaled or ingested.

Infected infants may display symptoms within 6 hours of birth or as late as 2 months of age. If untreated, the baby may become septic, develop pneumonia, suffer hearing or vision loss, or develop varying degrees of physical and learning disabilities.

To best assess the risk of a pregnant woman infecting her baby at delivery, the U.S. Centers for Disease Control and Prevention (CDC) recommend that all pregnant women be screened for GBS between 35 and 37 weeks of gestation. Specimens from the mother’s vaginal and rectal areas are collected, and within 24 to 48 hours the laboratory can determine if Group B strep bacteria are present. If the bacteria are present, or if the woman goes into labor before testing can be completed, it is recommended that she receive antibiotics intravenously during labor.

GBS may be present in a woman’s vagina off and on throughout her pregnancy. Therefore, testing for GBS earlier in her pregnancy is not useful in determining whether or not it will be present when she goes into labor and might risk spreading it to her baby during delivery. Only testing late in the pregnancy, between 35 and 37 weeks, is useful. Likewise, treatment with oral antibiotics before labor has not proven to be effective in preventing GBS infections in the newborn.

Complete blood count (may be repeated)

The complete blood count (CBC) is a test that evaluates the cells that circulate in blood. Blood consists of three types of cells suspended in fluid called plasma: red blood cells (RBCs), white blood cells (WBCs), and platelets (PLTs). To identify and prevent problems, a CBC may be done before pregnancy, if possible, at the beginning of pregnancy, and one or more times during pregnancy. The first baseline results can be compared to later values to look for changes that could indicate a health issue.

  • Red blood cells contain hemoglobin, a protein that gives blood its red color. It binds to oxygen in your lungs, transports it throughout the body, and releases it to the cells and tissues. During pregnancy, a woman’s hemoglobin must transport enough oxygen to meet both her and her fetus’ needs. If a woman has insufficient RBCs and/or hemoglobin, she is said to be anemic.

    Many pregnant women will experience some degree of anemia. While mild anemia can make you feel tired and weak, more severe anemia in a pregnant woman can cause a fetus to receive too little oxygen to support normal development.

    All women lose a small amount of blood during delivery. Although this is usually not a problem, even a small amount of blood loss can be harmful to women with anemia. A healthcare practitioner may want to know the level of hemoglobin in a pregnant woman’s blood before delivery to assess the possible impact of the expected blood loss.

  • White blood cells help protect the body from infections and also have other immune functions. Evaluating WBCs during a woman’s pregnancy can help detect infections so that they may be treated and resolved before they cause significant health problems in the mother or her baby.
  • Platelets are special cell fragments in the blood that help form clots to stop bleeding. Women with low platelet counts, or who have platelets that don’t function properly to form clots, are at risk of life-threatening bleeding during delivery. Follow-up testing may be needed to help determine treatment options if a platelet problem is detected.

Thyroid stimulating hormone if a woman has a history of thyroid disease

Pregnancy causes normal changes in the function of many endocrine glands, but it has a marked effect on the thyroid gland, which produces hormones such as thyroxine (T4) and triiodothyronine (T3) that are critical to the healthy development of a fetus as well as the health of the mother.

Women with known thyroid conditions will usually require careful monitoring if they become pregnant. A healthcare practitioner may use tests for thyroid stimulating hormone (TSH) to monitor a woman’s thyroid function throughout her pregnancy. TSH is produced by the pituitary, a small gland in the brain, in response to low T4 or T3 levels. Increased TSH levels in women who are taking thyroid hormone replacement may mean that their dose of hormone replacement needs to be increased.

Some have advocated screening pregnant women during the first trimester (or even before pregnancy) for elevated TSH even if they do not have a history of thyroid disease. This is because a significant percentage of women may have an underlying thyroid disorder that is unsuspected and that will cause problems during pregnancy. However, most guidelines do not recommend this as being necessary.

RBC antibody screen

Blood types are either A, B, AB, or O, and Rh positive or negative. A pregnant woman should know her blood type. [See Blood Typing for more information on how this is performed.] This is because both the mother and her baby may experience problems if their blood types are different from each other, or if the mother has antibodies (antiglobulins) that react with antigens (proteins or factors) on the fetus’ red blood cells. This may result in a serious condition referred to as hemolytic disease of the newborn (HDN).

The best known example is when an Rh-negative woman is pregnant with an Rh-positive baby. The woman’s immune system can develop an antibody that attaches to the Rh-positive antigens on her fetus’ red blood cells and target them for destruction. Although the first Rh-positive baby is unlikely to become ill, the antibodies produced during that first pregnancy will affect future Rh-positive babies.

To greatly reduce the likelihood that an Rh-negative mother will develop this antibody, she may be given a routine injection of Rh immune globulin (RhoGam) at approximately 28 weeks’ gestation. Additional injections may be necessary during the pregnancy if she has an amniocentesis, chorionic villus sampling, or an abdominal injury, and after delivery if the baby is Rh-positive. Before each injection is given, an antibody screen is performed to make sure that the woman has not already created Rh antibodies.

In addition to Rh-negative women who have had an Rh-positive baby, any woman who has had a blood transfusion or had prior pregnancies may produce an antibody to blood factors other than Rh that can potentially harm an unborn baby. An antibody screen during the first trimester determines if potentially harmful antibodies are already present in the mother’s blood. If a harmful antibody is detected, the baby’s father should be tested, if possible, to see if his blood has antigens that react with the mother’s antibody. If it does react, then the fetus’ may also. If the antibody could react with the fetus’, the healthcare practitioner should monitor the mother’s antibody level and the fetus for the duration of the pregnancy. Signs that the fetus is becoming ill may necessitate treatment before birth (such as an intrauterine transfusion) or an early delivery.

Although Rh incompatibility has more severe consequences, one of the most common causes of HDN is actually an incompatibility between the mother’s and baby’s ABO blood groups, not the Rh factor. However, the RBC antibody screen cannot be used to predict whether HDN will occur because antibodies to the ABO blood groups are naturally-occurring.

Fetal fibronectin (fFN) if a woman has symptoms of preterm labor

Fetal fibronectin (fFN) testing is performed if a woman is 22 to 35 weeks pregnant and having symptoms of premature labor to predict the risk of premature delivery. The goal then is to intervene to prevent the potentially serious health complications of a pre-term baby.

A cervical or vaginal fluid sample is collected and analyzed for fFN, a glycoprotein found at the boundary between the amniotic sac and the lining of the uterus. High levels can be due to causes other than risk of pre-term delivery; therefore, a positive fFN result is not very predictive of preterm labor and delivery. However, a negative fFN result is highly predictive that preterm delivery will notoccur within the next 7-14 days. Since there are risks associated with treating a woman for premature labor (in anticipation of a premature delivery), a negative fFN can reduce unnecessary hospitalizations and drug therapies.

Amniocentesis if risk of preterm labor

Amniocentesis may be performed after 32 weeks of gestation to help assess the degree of lung development for babies at risk of premature delivery.

During the procedure, a needle is inserted through the walls of the abdomen and uterus and into the thin-walled sac of fluid that surrounds the developing fetus. A small amount of amniotic fluid is withdrawn. This fluid contains AFP produced by the baby and fetal cells. The fetal cells can be tested for chromosomal or genetic abnormalities. A gene analysis may be performed based on family history (i.e., another child born with a defect or with a hemoglobinopathy) or on the results of screening tests performed on the parents (such as for cystic fibrosis). Approximately two weeks are needed to complete the testing.

There is a slight risk with amniocentesis that the needle inserted into the amniotic sac may puncture the baby, cause a small amount of amniotic fluid leakage, cause an infection, and in rare cases cause a miscarriage.

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(July 2008) U.S. Preventive Services Task Force. Asymptomatic Bacteriuria in Adults: Screening. Available online at Accessed October 2015.

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For additional sources, see the overview article on Pregnancy.