Pregnancy: Second Trimester (13 to 27 weeks)
The laboratory tests performed during the second trimester of pregnancy provide information to evaluate both actual and potential medical problems in the baby while continuing to monitor the mother’s health during the pregnancy.
Many women have an ultrasound diagnostic imaging test as part of their routine prenatal care, often between weeks 18 and 20. This non-invasive test, called a sonogram, may also be useful earlier, later, or again during the pregnancy to look for an indication of problems with the baby’s organs, such as the heart and lungs. It may also be used to help determine how far along a woman is in her pregnancy and help assess the baby’s growth and health. Ask your healthcare practitioner what is appropriate in your situation.
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 test, full urinalysis, or urine culture (to identify any bacteria or yeast present; see Bacteriuria below).
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 (bacteriuria)
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 pre-term delivery and low birth weight. Treatment with appropriate antibiotics is advised.
Blood glucose and glucose tolerance test for gestational diabetes
During pregnancy, some women develop gestational diabetes (increased glucose levels). Although it can occur at any time, most cases will develop during the later part of the pregnancy. If increased blood sugar levels in the pregnant woman are uncontrolled, they can cause the fetus to increase in size and weight. They can also cause the baby to be born with very low glucose levels and to have breathing difficulties.
Most women are checked for gestational diabetes between 24 and 28 weeks of pregnancy. Sometimes a test for diabetes is done earlier in the pregnancy if a woman is suspected of having pre-existing diabetes or is considered to be at higher than average risk.
Risk factors for diabetes include:
- Older age
- Ethnicity with higher rate of diabetes such as Hispanic, Native American, South or East Asian, African American, or Pacific Islands descent
- Family history of diabetes
- Personal history of gestational diabetes with a previous pregnancy
Several health organizations recommend that all pregnant women be screened for gestational diabetes after 24 weeks of pregnancy.
The American College of Obstetricians and Gynecologists (ACOG) recommends screening using patient history, risk factors, or laboratory testing.
In the U.S., laboratory testing may involve either a one-step or two-step approach:
- One-step 2-hour oral glucose tolerance test (OGTT): After a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values needs to be above a cutoff value for diagnosis.
- Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
- If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman’s fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made.
ACOG and the National Institutes of Health consensus conference currently recommend the two-step approach, the Endocrine Society recommends the one-step approach, and the American Diabetes Association (ADA) says either the one-step or two-step approach is appropriate.
For women with gestational diabetes, diet control and/or insulin injections throughout the rest of the pregnancy may be required to bring glucose levels down to normal levels. In most cases, gestational diabetes will go away after delivery, but women who have gestational diabetes will be at an increased risk of having it again with subsequent pregnancies and of developing diabetes in the future. Some organizations, including ACOG and the ADA, recommend that women diagnosed with gestational diabetes be screened 6-12 weeks after delivery for diabetes that persists.
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.
Tests for Fetal Abnormalities
Maternal serum screen
Between 15 and 20 weeks’ gestation, the mother may be given the option to have a maternal serum screening test for Down syndrome and open neural tube defects, such as spina bifida. Included in the screening panel are tests for alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol, and sometimes inhibin A. (Depending the number of markers used, the test may also be called a triple marker or a quad marker test.) This blood screening panel provides an opportunity to assess the risk of these conditions occurring without performing the more invasive amniocentesis procedure. In most cases, only those patients whose blood screening results indicate a potential problem need to be referred for amniocentesis.
During the second trimester, it is expected that the levels of AFP and unconjugated estriol will increase, the amount of hCG will decrease, and the amount of inhibin A will stay relatively constant.
- AFP is produced by the fetus and then crosses into the mother’s blood. A fetus with a neural tube defect has an opening in its spine or head that allows an increased amount of AFP to pass into the mother’s blood stream. An elevated AFP could also result from multiple fetuses, miscalculation of gestational age, an abdominal wall defect, or an unknown reason. An ultrasound may be requested to determine the fetus’ age and confirm the number of fetuses.
- In pregnancies where the fetus is carrying the chromosomal defect that causes Down syndrome, the results of the triple or quad marker screen tend to show decreased levels of AFP and unconjugated estriol and increased levels of hCG and inhibin A.
The test results are interpreted based on the mother’s age, weight, and ethnic background to assess the risk of the baby having neural tube defects or a chromosome disorder. Of all reported cases of elevated AFP, only a very small percentage of babies truly have a defect. But if the test result is abnormal, a genetic counselor should be consulted to discuss an amniocentesis to further assess the likelihood of a birth defect.
The test is most often offered between 15 and 20 weeks of gestation (usually around week 16) to a pregnant woman under any of the following circumstances:
- She is 35 years of age or older.
- There is a strong family history of a genetic disorder from the parents.
- Both parents possess a gene for an inherited disorder.
- Results of screening tests indicate increased risk of fetal abnormality.
- A previous child in the family had a birth defect or chromosome problem.
Amniocentesis may also 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.
Cordocentesis (percutaneous umbilical blood sampling, PUBS)
Percutaneous Umbilical Blood Sampling (PUBS) is a procedure that can be used to obtain a small sample of the fetus’ blood. The blood can then be analyzed for certain abnormalities. This diagnostic test (also called cordocentesis, fetal blood sampling, or umbilical vein sampling) is used to detect chromosome abnormalities indicating Down syndrome and blood disorders such as anemia and certain infections. PUBS is used when amniocentesis, chorionic villus sampling, or ultrasound cannot provide the information being sought or when these tests yield inconclusive results. PUBS can also be used as a way to deliver therapy to the fetus.
Cordocentesis is performed in a manner similar to amniocentesis. A fine needle is inserted through the pregnant woman’s abdomen into the vein of the umbilical cord. The procedure lasts about an hour and may be done between weeks 18 to 22 of the pregnancy.
Miscarriage occurs 1 to 2 times out of every 100 procedures. Other potential side effects include blood loss from the puncture site, infection, a drop in the fetus’ heart rate, or premature rupture of the membranes.
Cell-free fetal DNA (non-invasive prenatal testing, NIPT)
Cell-free fetal DNA (cffDNA) testing is a non-invasive maternal blood test that can assess the risk of a pregnant woman’s developing baby (fetus) having certain chromosome disorders early in pregnancy. The test requires only a blood draw from a pregnant woman’s arm and can be done as early as the 10th week of pregnancy, with results available in about one week. The blood sample is evaluated for fetal cell-free genetic material that circulates in the mother’s blood.
Currently, the test primarily screens for three disorders in a developing baby resulting from the presence of an extra chromosome (trisomies): Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). It may also be used to identify other rare conditions resulting from an extra chromosome or missing piece of chromosome (microdeletion).
Studies of cffDNA have shown an ability to recognize 98% of cases of Down syndrome, with a false-positive rate of less than 0.5%. The current first trimester screen only assesses risk of Down syndrome and Edwards syndrome, whereas cffDNA screens for three trisomy disorders with the potential for more.
The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women discuss the risk and benefits of prenatal screening with their healthcare practitioners and/or a genetic counselor. Conventional screening such as second trimester maternal serum screening remains the best choice for women with a low risk of having a baby with a chromosome disorder, says ACOG. However, any woman may choose to have cffDNA testing done regardless of her risk.
ACOG recommends that cffDNA testing be offered, after pre-test genetic counseling, to pregnant women at increased risk. They include women age 35 and older, women with a history of a pregnancy with a child with a trisomy, and women carrying a fetus with ultrasound findings indicating increased risk.
Women should make this choice with a thorough understanding of the limitations of the test. CffDNA will detect only the most common trisomies. False-positives and false-negatives do occur. A negative result does not necessarily mean that a woman will have an unaffected child. Women with positive cffDNA results should be referred for genetic counseling and be offered chorionic villus sampling (CVS) or amniocentesis to confirm the diagnosis.
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For additional sources, see the overview article on Pregnancy.