A first trimester screening is an examination method used to estimate a possible chromosomal aberration in a fetus. The screening involves biochemical blood analysis of the pregnant woman and ultrasound examination of the unborn baby. First trimester screening is not used to determine a definite diagnosis, but only to assess risk.
What is first trimester screening?
Screening is done in the first trimester (the first third of a pregnancy) to detect a possible chromosomal abnormality in a fetus. First-trimester screening is a systematic investigation to screen out abnormalities that indicate a prenatal likelihood of risk. Screening is performed in the first trimester (the first third of a pregnancy) to detect a possible chromosomal abnormality in a fetus. A chromosomal abnormality can thus be diagnosed as early as the first 3 months of the 9-month pregnancy in the form of trisomy 21 through further testing. First trimester screening is a non-invasive procedure to detect an increased risk of trisomy 21 (Down syndrome) in the unborn child and is a safe examination in terms of detection with a high detection rate. As an alternative diagnostic procedure to first trimester screening, so-called “integrated screening” and “sequential screening” are also frequently performed. In first trimester screening, two biochemical values are determined and assessed from the mother’s blood. Furthermore, a sonography of the fetal nuchal translucency is performed and measured. Nuchal translucency is an accumulation of fluid between the skin and soft tissues in the cervical region in the unborn child. The mother’s medical history is added to these results. Based on this information, the attending specialist can assess the result and weigh a probability of risk. However, the resulting prognosis should not be confused with a confirmed diagnosis. Further clinical tests are necessary to diagnose Down syndrome.
Function, effect, and goals
Prior to first-trimester screening, expectant parents should consider the extent to which estimating a possible chromosomal abnormality is helpful and what consequences it may have for the subsequent course of the pregnancy. The probability calculation is based on the age of the pregnant woman, the week of pregnancy and any existing chromosomal abnormalities within the family. Furthermore, the results of the nuchal fold measurement, the concentration of the protein PAPP-A and the hormone ß-hCG (human chorionic gonadotropin), representation of the nasal bone in the fetus and the blood flow in the heart and in the large blood vessels of the unborn child are evaluated. Biochemical evaluation of laboratory values involves analysis of the concentration of the protein PAPP-A and that of the hormone ß-hCG in maternal blood. Pregnancy-associated plasma protein A (PAPP-A) is a zinc-binding protein and functions like an enzyme. If the concentration of PAPP-A in maternal blood is too low, this may indicate intruterine growth retardation. Human chorionic gonadotropin (hCG) is a pregnancy hormone that is secreted in the mother’s body shortly after fertilization. The subclass of ß-hCG is specific for the hormone and has 145 amino acids. If these values deviate from the statistical norm during the 11-13 weeks of pregnancy, the risk of an abnormality increases. In combination with the measurement of fetal water retention in the cervical region by ultrasound, this equates to a large number of already known normal values of pregnancies with positive and negative diagnoses. This allows the weighing of a possible fetal chromosomal aberration, but only as a risk assessment. However, this estimation modifies an already existing risk pregnancy in case of advanced maternal age or recurrence after previous pregnancies with fetal Down syndrome. All values of the anamnesis and the first trimester screening are evaluated by a special computer program and finally assessed by the specialist. If the specialist determines that the threshold values have been exceeded and thus that there is an increased probability, a chorionic villus sampling or amniocentesis (amniocentesis) should be performed to provide clarity.The advantage of a chorionic villus sampling is that it can be performed earlier than an amniocentesis. However, both types of examination are invasive procedures that carry risks for the pregnant woman and the unborn child. The risk of miscarriage during such an examination is about 0.3 – 1 %. First trimester screening detects 95 out of 100 unborn babies with Down syndrome and thus has a 95 percent significance. Nevertheless, 5 out of 100 healthy unborn babies are also falsely detected with an increased risk probability of trisomy 21.
Risks, side effects, and hazards
The collection of blood and sonography during first trimester screening is safe for the pregnant woman and for the fetus. The actual consequences result from the theory of risk assessment that first trimester screening produces. Screening does not provide a definite finding and this can lead to uncertainty or even wrong decisions by the expectant parents. Furthermore, there are many factors that influence the value of the concentration in the blood, making the result useless. In multiple pregnancy, the level of pregnancy hormone ß-hCG and Pregnancy-associated plasma protein A (PAPP-A) is basically increased. Also, women who smoke or eat a vegetarian/vegan diet during pregnancy have elevated levels of ß-hCG, even though the unborn baby is healthy. In addition, an inaccurately calculated duration of pregnancy, obesity and diabetes mellitus in the pregnant woman can influence the values. Furthermore, delayed development of the fetus, placental insufficiency, and renal insufficiency of the expectant mother may be causal factors for a falsified result. If there is sufficient evidence of an increased risk, this should be investigated with certainty using amniocentesis or a chorionic villus sampling. If there is insufficient evidence, such a high-risk procedure should not be performed.
