Prenatal diagnosis

Guide for Pregnant Patients: Prenatal Diagnosis in the First Trimester

During the first trimester of pregnancy, various tests are available to assess the risk of chromosomal abnormalities in the baby. These tests help detect conditions such as Down syndrome, Edwards syndrome, and other genetic abnormalities. Below, we explain the main available techniques.

1. Biochemical Screening Combined with Ultrasound

This study assesses the risk of chromosomal abnormalities by combining a maternal blood test and a specialized ultrasound.

• Nuchal translucency ultrasound: Performed between weeks 11 and 14 of pregnancy. It measures the thickness of the crease at the back of the baby's neck, which may indicate an increased risk of genetic abnormalities.
• Maternal blood test: Two biochemical markers (free β-hCG and PAPP-A) are measured at weeks 9-10 of pregnancy.
• Results: Does not diagnose, but estimates risk. It has a sensitivity of 90% and a false-positive rate of 4%. If the risk is high, additional testing may be considered.

Risk estimates for T21 and T18/T13 are based on the risk inherent to maternal age, modified by the deviation of the three markers (nuclear translucency, BHCG, PAPP-A) from the expected values for gestational age. In the case of oocyte donation, the maternal age to be considered will be that of the donor. Risks for T21 or T18/T13 are classified into four levels:

- Very high: between 1/2 and 1/10.

- High: between 1/11 and 1/250.

- Intermediate: between 1/251 and 1/1100.

- Low: <1/1100.

In cases of very high risk, invasive testing (chorionic biopsy or amniocentesis) should be considered. In cases of high or intermediate risk, fetal DNA testing or invasive testing (chorionic villus sampling or amniocentesis) may be offered. The advantage of cell-free DNA testing is avoiding the risk of fetal loss, and the disadvantage is that it provides information restricted to the chromosomes studied and is less accurate, although the detection rate for trisomy 21 is 99%. The disadvantage of invasive diagnostic testing is that it presents a 0.1-0.2% risk of fetal loss, and the advantage is that it provides highly accurate information on all chromosomes.

2. Fetal DNA testing in maternal blood

This is a more advanced screening test that analyzes fetal DNA fragments present in maternal blood.

• When it is performed: From week 10 of gestation.
• Accuracy: It has a high sensitivity (approximately 99%) for detecting common trisomies such as Down syndrome.
• Advantages: It is noninvasive and poses no risk to the mother or baby.
• Limitations: It is not diagnostic, but rather indicates probabilities. If the result is high-risk, confirmation with an invasive test is recommended.

3. Invasive Tests: Chorionic Gonadotropin and Amniocentesis

These tests provide a definitive diagnosis by directly analyzing fetal cells.

• Chorionic Gonadotropin: Performed between weeks 10 and 13. A small sample of the placenta is removed for DNA analysis.
  • Advantages: Early diagnosis.
  • Risks: Slight risk of miscarriage (approximately 0.5-1%).
• Amniocentesis: Performed from week 15 onwards. It involves removing a small amount of amniotic fluid containing fetal cells.
  • Advantages: Highly accurate results.
  • Risks: Low risk of complications (0.1-0.3% miscarriage rate).

Each of these tests has its advantages and limitations. The choice of which test to undergo depends on individual factors, medical history, and each patient's preferences. We understand this can be an important decision, and we want you to know that you are not alone in this process. You can count on us to discuss all the options, answer any questions, and support you every step of the way through your pregnancy. Don't hesitate to consult with us to make an informed decision about the best approach for you and your baby.