There are a number of tests that are useful in pregnancy.  These fall into three categories: diagnosis of pregnancy, screening for risk to certain conditions, and monitoring the progress of the pregnancy.

Diagnosis of Pregnancy

The diagnosis of pregnancy is eventually obvious.  Early on, pregnancy testing can be used to detect pregnancy before clinical features appear.  We no longer use rabbits or frogs for pregnancy testing but rely on immunoassay kits. Modern pregnancy tests carried out in the urine are extremely sensitive and can detect the presence of a pregnancy before the time when a period would begin.  There are a few causes of false positive and false negative results.  The most common errors are caused by people using home kits.  Sometimes they do not use them or read the results properly or may have stored them inappropriately.  The lab uses similar methods but the test is performed by skilled lab technologists who use stringent control techniques.  Nevertheless, aberrant results do occur. Apparently false positive results occur when a woman actually becomes pregnant but the pregnancy terminates at a very early stage.  This happens when the egg becomes fertilized late in the menstrual cycle and although it implants itself in the uterus (thereby causing the pregnancy test to turn positive) the menses have already started and cause the pregnancy to be lost. The test may even stay positive for some time after the termination as it takes time for the hormone that is measured (hCG) to be removed from the blood stream.  This is a more common event than most people realize.  Other causes of  “false positive” results are Hydatidaform mole or certain types of cancer. False negative results may occur if the test is performed too early or if there is a very large volume of urine that dilutes out the hCG.

HCG can be measured in the blood.  Theoretically, this is an even more sensitive assessment but in practice, this test becomes positive only hours before the urine turns positive.  It does not suffer from dilution problems.  The best use of blood measurements is in the diagnosis of ectopic pregnancy and the diagnosis and treatment of cancers such as choriocarcinoma and mole.  It is also part of the Triple Marker test described below. Rarely, a person may have an antibody in her blood that interferes with the test.  Such persons may have an apparent increase in hCG  and because they are not pregnant they may be misdiagnosed as having a certain type of hCG producing cancer.  It is important to evaluate such cases using ultrasound and an hCG assay that employs a different antibody.  This can be arranged by the lab.

Screening

Early in pregnancy, blood is collected and sent to the Red Cross and the Center for Disease Control in Vancouver.  Here, testing for Rubella (German Measles), Syphilis, Blood Group and Rh type, and Hepatitis is carried out.  At 24 – 28 weeks a Glucose load is given as a screen for gestational Diabetes (see Diabetes).

The Triple Marker Test

The triple marker screen is a blood test performed at about the 15th week of pregnancy. It is called “triple” marker because there are three substances that are measured in the mother’s blood.  The values obtained on these measurements can be used to determine increased risk of the baby having certain conditions.  The blood test will rule out the possibility of these disorders (with a very high probability but not 100%).  A positive test does not indicate a problem.  It only tells your doctor that a problem cannot be ruled out.  Additional testing is therefore required to be certain. The vast majority of women with a positive Triple Marker Screen are subsequently shown to be normal.  The screen is useful because it reduces the number of mothers who need to have the more rigorous test.

What does the Triple Screen tell your doctor? One problem area is in the development of the fetus' brain, spinal cord and other neural tissues of the central nervous system (neural tube). Problems with neural tube development can occur as spina bifida or anencephaly. Neural tube defects occur in 1 or 2 out of every 1,000 births. The Triple Marker screen is positive in approximately 75 percent of open neural tube defects. Genetic disorders such as Down syndrome, a chromosomal abnormality, occurs in approximately 1 in 720 births. The triple marker screen can detect approximately 70 percent of Down syndrome cases in women under age 35 and 85 to 90 percent of Down syndrome cases in women age 35 years and older

The blood sample is sent to a laboratory and tested for the presence of the following three substances which are normally found in the baby's blood, brain, spinal fluid and amniotic fluid:

Alpha-fetoprotein (AFP) is a protein produced by the fetus' liver. Higher than normal AFP levels could indicate that the fetus has an open neural tube defect. High AFP levels may also indicate that the fetus is older than was thought or that the woman is expecting twins. Lower than normal AFP levels could indicate that a woman is at higher risk for having a baby with Down syndrome.

Unconjugated Estriol (UE)  is a steroid produced in the placenta and in the fetus' liver. Lower than normal levels of Estriol may also indicate that a woman is at high risk for having a baby with Down syndrome.

Human Chorionic Gonadotrophin (hCG) is a hormone produced by the placenta. Levels of hCG are higher than normal when a woman has an increased risk of having a baby with Down syndrome.

The expected amount of these substances normally found in the mother's bloodstream changes each week of pregnancy. The results of the test are compared to these expected ranges and to data from patients with problem pregnancies to obtain an overall risk factor and a recommendation for further testing.

Normal levels of AFP, UE and HCG strongly indicate that you have a healthy pregnancy and a healthy baby. In over 98% of pregnancies, normal triple marker screen results predict healthy babies and births without major complications. However, there are no prenatal tests that can guarantee your baby and pregnancy will be completely healthy or without complications.