What is prenatal screening?
Prenatal risk screening consists of a series of tests aimed at detecting any possible birth defects in uterus. These refer to structural, molecular, functional or morphological developmental abnormalities present at birth, even if they develop in the future. It does not matter whether these possible defects are external or internal, hereditary, sporadic or familial, multiple or single. It should be noted that these tests are not diagnostic of a fetal anomaly, but indicate whether the risk is normal or elevated.
Prenatal risk screening also analyzes the risks of chromosomal abnormalities, thanks to the screening of low-risk pregnancies. In the case of high-risk pregnancies, counseling is performed. In order to carry out cytogenetic studies, diagnostic procedures are performed.
The purpose of genetic counseling (or reproductive counseling) is, in short, to quantify the risk of an unfavorable outcome. Thus, the most appropriate counseling and the next steps to take in order to prevent it can be offered. This process, which can be either pre- or post-conception, consists of several phases:
- Collection and analysis of family, personal and reproductive history to determine the specific cause of the maternal-fetal reproductive risk.
- Selection of specific information regarding the risks of the fetal anomaly. Data on the mechanisms of interference are also necessary, communicating this carefully to the couple and helping to reduce the distress surrounding reproduction.
- It is necessary to discuss with the couple the pros and cons of various alternative, non-directed methods.
- A prevention plan (primary or secondary) is established.
Why are screening tests performed in the first and second trimester?
Prenatal risk screening tests are divided into 2 parts: some are performed in the first trimester and others in the second trimester. These tests are used to analyze the risk of the patient being a carrier of a fetus with chromosomal abnormalities, such as Edwards’ syndrome (Trisomy 18) or Down syndrome (Trisomy 21).
These tests are not mandatory, but highly recommended.
First trimester screening tests.
During the first trimester, prenatal risk index for Edwards’ syndrome and Down syndrome is the most popular method. This is because the detection rate is between 75-85%. The chances that the fetus may be affected by either of these 2 diseases (T21, T18) are analyzed from the risk inherent to the maternal age. These can be modified by deviation in first trimester ultrasound and biochemical markers. The procedure is as follows: all pregnant women, regardless of age, who are not at increased risk of parental chromosomal abnormality and who consult before 13.6 weeks, are routinely screened. An ultrasound scan is performed, if possible at 2 different gestational periods. If this is not the case, both ultrasounds can be performed on the same day.
Blood collection should be performed between 7.6 and 13.6 weeks due to amenorrhea, although ideally at 8-10 weeks. The patient does not need to fast or have a prior dating ultrasound. The laboratory will determine the levels of free β-fraction of chorionic gonadotrophin (fβ-hCG) and pregnancy-associated placental protein A (PAPP-A). After reporting the ultrasonographic gestational age, the values will be expressed in MoM (median multiple).
Ultrasonography is performed between 11.2 and 13.6 weeks (CRL 45-80). It is best performed at 12 weeks in order to date the gestation, rule out multiple gestation and assess nuchal translucency (NT). In this case the 50th percentile of the Robinson & Fleming (1975) table is used as the only dating table from the CRL.
Outside the range 45-80 mm CRL the following steps occur:
- CRL << 45 mm. In this case, ultrasonography is rescheduled. Biochemistry will have to be repeated if it was drawn before 7.6 weeks.
- CRL 81-84 mm: this is feasible if the blood collection was performed before 14.0 weeks (up to a CRL 80 mm).
- CRL >> 84 mm: second trimester screening is necessary.
If the ultrasound is performed on the same day as the blood draw, this ultrasound data should be provided to the health care provider. If the ultrasound is scheduled for after the analysis, it is mandatory to provide these data to the laboratory.
The values obtained for CRL and NT from ultrasound should be entered into a risk calculation software. These values and the biochemical results will tell us the prenatal risk index for both syndromes. These results take into account data such as the mother’s ethnicity or weight, as well as whether or not she is a smoker or insulin-dependent. With all these data, we can finally tell the pregnant woman what her prenatal risk probabilities are.
Second trimester screening tests.
During second trimester prenatal risk screening, with a lower detection rate than in the first trimester (75%), the risk for T21 and T18 is estimated in relation to the risk of the mother’s age at delivery modified by the deviation of second trimester biochemical markers. Regardless of age, all mothers are routinely screened from 14.0 weeks onwards. For this, a maternal blood draw and ultrasound are performed with the same characteristics as in the first trimester: preferably in 2 different seasonal periods. If this is not possible, both tests are performed on the same day.
The blood draw is performed between 7.6 and 13.6 weeks due to amenorrhea, but it would be perfect at 8-10 weeks. The patient does not need to fast but, unlike the first trimester, it is not necessary to have a prior dating ultrasound. The objective will be to determine the levels of free β-fraction of chorionic gonadotrophin (fβ-hCG) and pregnancy-associated placental protein A (PAPP-A). As in the first trimester, risk distribution will be done, accompanied by counseling at each level. The difference is that an amniocentesis should be performed as an invasive procedure.
To date the gestation and rule out multiple gestation, an ultrasound scan is required. The buccal fold will be assessed when NT in the first trimester has not been assessed. At the time it is considered, the nuchal fold will be incorporated as a new marker for second trimester biochemical screening.
What happens during screening?
There is a statistical computer program that is responsible for relating the different biochemical and ultrasound parameters, in addition to other clinical data of the pregnant woman. The objective of this analysis is to calculate the possibility of risk in the triple screening. All this information is obtained with the following tests:
- Blood tests. The values of placental protein associated with pregnancy (PAPP-A) and free beta-hCG hormone are obtained.
- Ultrasound. The purpose of ultrasound at this stage is to measure the fetal nuchal fold or nuchal translucency (NT).
- Clinical information. Data on the mother, such as weeks of pregnancy, number of fetuses, age, etc.
Between 75-85% of cases are detected with combined screening. False positives are usually around 3%, but it is highly recommended to perform the test to identify and rule out chromosomal pathologies.
When are first and second trimester screening tests performed?
This test is optional and is offered to all pregnant women who go for prenatal consultation in the 14 weeks prior to pregnancy. As mentioned above, the tests are performed between 8-13 weeks of pregnancy. The methods used are one of the following:
- Combined 1-time screening. The blood draw and ultrasound are performed on the same day (between 11 and 13+6 weeks).
- Combined 2-stage screening. The blood draw is more relaxed; it can be performed at any time between 8 and 13+6 weeks of pregnancy. However, ultrasound should be performed between 11 and 13+6 weeks of pregnancy.
Both options are perfectly valid, and depend on the preference of the pregnant woman. If the pregnant woman comes for her first prenatal visit before 8 weeks, the combined screening can be scheduled at two different times. In this way, the first blood test is used to determine biochemical markers, in addition to serology and blood grouping. Weeks 15 and 20 of gestation are suitable for second trimester screening. Second trimester screening is not usually performed if the pregnant woman has successfully passed the first screening.
When are screening test results available?
Prenatal risk screening tests are usually available 1 week after all the necessary data (blood tests and ultrasound) have been obtained. A specialist will be in charge of analyzing all the data obtained and reporting the results and possible solutions.
