Your health-care provider may order a variety of blood tests during your pregnancy. Blood tests are done with one needle prick, and one or more vials of blood are drawn. If you have difficulty having blood drawn, ask to lie down. Choose a focal point in the room or have someone with you who can distract and support you.
The first time you see your health-care provider for a prenatal checkup, it is likely he will ask you to get some blood work done. These tests will give your provider information about your blood type, Rh factor (negative or positive blood type), anemia, syphilis, hepatitis B, HIV, immunity to things like measles and chicken pox, and cystic fibrosis. Sickle cell anemia testing is usually ordered for women of African or Mediterranean descent. Special tests may be ordered for women of certain cultural or ethnic backgrounds (such as Tay Sachs disease testing for Jewish women, and thalassemia testing for women of Mediterranean background). Testing may also be done for hepatitis C if the history warrants. Some obstetrical caretakers may screen for toxoplasmosis (parasitic infection carried by cats or caused by eating partially cooked meat or raw vegetables), or cytomegalovirus, though these are not routinely tested for in the United States. If you have a profession that puts you in contact with young children, such as teaching, your obstetrical caretaker may test your immunity to parvovirus, or Fifth's disease.
Screening for gestational diabetes is done using a blood test after you have fasted and then drunk a special glucose drink. About 15 percent of women test positive. Because this test does result in false positives, you'll be sent for a longer test in which several blood samples are drawn over a longer time period. Generally, 30 percent of the women sent for this test ultimately have gestational diabetes.
Urine tests are routinely performed at every prenatal visit to check the urine for protein (which can signal preeclampsia) and sugar (a possible indication of diabetes or gestational diabetes). You will either be asked to bring a sample with you in a sterile jar or provide the sample upon your arrival at the office.
The first round of testing takes place between twenty-four and twenty-eight weeks. If you are obese, had a previous baby that weighed over nine pounds, had previous adverse pregnancy outcome, have glucose in your urine, have had gestational diabetes or diabetes in the past, or have a strong family history of diabetes, you may be tested in the first or second trimester and will have a repeat test at twenty-four to twenty-eight weeks if the initial screen is negative. There is some debate about whether all women should be screened for gestational diabetes or only those with certain risk factors. However, there is general consensus that women over 25 and certainly over 35 should undergo routine screening. Many obstetrical caretakers will substitute other testing substrates, such as soda or candy, for the standard glucose testing solution that you drink for the test. As a general rule, only the standard testing solution is considered accurate for screening.
Other Blood Tests
Other blood tests done during pregnancy are used to screen for birth defects and abnormalities in the fetus. A first-trimester assessment is a blood test that screens for trisomy 21 (the chromosomal abnormality known as Down syndrome) and trisomy 13 and 18 (other chromosomal abnormalities associated with advanced maternal age). Second-trimester testing includes AFP (alpha fetoprotein) and two additional hormones (triple screen) or three additional hormones (quad screen). The quad screen is currently considered the standard of care for second-trimester screening and is usually performed between fifteen and twenty weeks. It is best to screen as close to fifteen to sixteen weeks as possible so that timely testing can be performed if the test comes back positive. The quad screen assesses for trisomy 18 and trisomy 21, as well as spina bifida and other less common fetal abnormalities. A maternal serum AFP (MSAFP) may be performed alone, if the first trimester screen has already been performed and is negative, since the earlier screening tests cannot screen for spina bifida.
A quad screen does not screen for the chromosomal abnormality trisomy 13. This test is slightly less sensitive than a first-trimester blood test. Still, it is often used because in addition to trisomy 18 and 21, it can screen for spina bifida and other less-common birth defects.
If the quad test is positive, you may be offered the option of a genetic amniocentesis or a genetic ultrasound (as described on page 100–101). If the MSAFP is elevated, birth defects such as spina bifida can generally be detected by high-resolution sonography, and amniocentesis can usually be avoided.
It is important to understand the significance of a positive test result. These tests do not tell you that the baby does or does not have a chromosome problem. Instead, they give you an idea of the risk of specific chromosome problems, which may be high or low. A test only screens for the chromosome problems it was designed to detect (such as the sex chromosome trisomies described in Chapter 7). These tests detect a high percentage of babies with specific chromosome problems, such as Down syndrome, without resorting to invasive procedures. You should view a positive result only as an indication that you may need further testing. In the vast majority of cases, the baby is found to be normal.
The AFP or quad screen detects Down syndrome 85 percent of the time and has a 7 percent false positive rate. In other words, 7 percent of women are told their fetus has a problem when in fact it does not.
It is important to discuss screening with your health-care provider prior to the test so that test results can be placed in perspective and you can decide if you want to undergo screening at all. Remember, when the screen is negative or it shows a relative low risk, then it is reassuring. However, a positive screen may cause considerable anxiety and theoretically could result in the loss of a normal baby if an invasive procedure is opted for. You should ask your physician about your actual age-related risk prior to screening. You may find that risk quite acceptable in absolute terms and may not wish to undergo screening.
Percutaneous Umbilical Blood Sampling (PUBS) or Umbilical Vein Sampling
This sampling (PUBS) tests blood from the umbilical cord. Although the long name makes this procedure sound awfully dire, it is a very simple procedure in which a physician uses ultrasound to guide a needle into the umbilical vein to sample the baby's chromosomes and test the baby for anemia. This test is very quick, with the same sensation as amniocentesis and offers quick results. There is a 5 percent adverse occurrence rate; adverse occurrence includes miscarriage or emergency delivery for a complication in a baby who is viable.
This test is recommended for women who have an inconclusive amnio result, are exposed to infectious diseases during pregnancy, or have babies who have Rh disease. The test is performed at eighteen to thirty-six weeks, and results are available within three days. This test does have a higher complication rate than standard amniocentesis and requires a skilled and experienced individual to perform the testing.