Prenatal screening and diagnosis: Key tests and Procedures trimester by trimester

Prenatal screening and diagnostic tests are essential for monitoring the health of both the mother and baby during pregnancy. These tests help detect potential genetic conditions, birth defects, and other complications. Let’s break down the key tests and procedures trimester by trimester.

First trimester (weeks 1–13)

The first trimester focuses on early detection of potential genetic disorders and assessing the overall health of the pregnancy. Key tests and procedures during this stage include:

1. Blood tests and Urine tests

• Purpose: Detect the mother’s blood type, Rh factor, and screen for infections such as HIV, hepatitis, and syphilis. Hormone levels like hCG and progesterone are also monitored.
• Why It matters: These tests provide an overview of maternal health, screen for infectious diseases, and identify risk factors that could complicate the pregnancy.

2. Ultrasound (Dating scan)

• When: Around 8–12 weeks.
• Purpose: Confirms the pregnancy, estimates gestational age, and checks the baby’s heartbeat and location.
• Why It matters: Provides critical information on early development and confirms the due date.

3. Nuchal Translucency (NT) scan

• When: Between 11–13 weeks.
• Purpose: Measures the fluid at the back of the baby’s neck to assess the risk of Down syndrome and other chromosomal abnormalities.
• Why It matters: When combined with other blood tests, it helps evaluate the risk of chromosomal conditions.

4. Double marker test

• When: Between 10–14 weeks.
• Purpose: A blood test that measures two markers (free beta-hCG and PAPP-A) to assess the risk of chromosomal abnormalities such as Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18).
• Why It matters: This non-invasive test helps in the early assessment of potential genetic disorders.

5. Chorionic Villus Sampling (CVS)

• When: Between 10–13 weeks.
• Purpose: A diagnostic procedure where a small sample of cells from the placenta (chorionic villi) is taken to test for chromosomal abnormalities and genetic disorders.
• Why It matters: CVS provides definitive results for detecting conditions such as Down syndrome, cystic fibrosis, and other inherited disorders. It is typically recommended if the results of initial screenings indicate a high risk, or if there is a family history of genetic disorders.

6. Non-Invasive Prenatal Testing (NIPT)

• When: From 10 weeks onward.
• Purpose: A blood test that analyzes fetal DNA circulating in the mother’s bloodstream to screen for chromosomal conditions such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13.
• Why It matters: NIPT offers highly accurate, non-invasive screening for genetic conditions, especially in high-risk pregnancies, and is often used as a follow-up to other screening tests.

7. First-Trimester combined screening

• When: Between 11–14 weeks.
• Purpose: Combines the NT scan and the double marker test to assess the risk of chromosomal abnormalities.
• Why It matters: This combined test provides an early and accurate risk assessment for genetic conditions, guiding further diagnostic decisions.

Second trimester (weeks 14–26)

As the baby develops, the second trimester introduces more detailed testing, focusing on the baby’s growth and detecting any potential issues.

1. Anomaly scan (Mid-pregnancy ultrasound)

• When: Between 18–22 weeks.
• Purpose: A detailed scan that examines the baby’s anatomy, including the brain, heart, spine, and organs. It also evaluates the placenta and amniotic fluid.
• Why It matters: This scan helps detect structural abnormalities such as spina bifida, heart defects, and limb malformations.

2. Amniocentesis

• When: Between 15–20 weeks (if indicated).
• Purpose: A diagnostic test where a sample of amniotic fluid is taken to test for chromosomal abnormalities, genetic disorders, and neural tube defects.
• Why It matters: Amniocentesis provides a definitive diagnosis if the screening tests show a high risk for genetic disorders.

3. Glucose Tolerance Test (GTT)

• When: Around 24–28 weeks.
• Purpose: Screens for gestational diabetes by testing how the mother’s body processes glucose.
• Why It matters: Detecting gestational diabetes early helps manage it through diet and medication to reduce complications for both mother and baby.

Third trimester (weeks 27–40)

In the final trimester, the focus shifts toward monitoring the baby’s growth, ensuring the pregnancy remains healthy, and preparing for birth.

1. Group B Streptococcus (GBS) screening

• When: Around 35–37 weeks.
• Purpose: A swab test to detect the presence of Group B Streptococcus bacteria in the vaginal area.
• Why It matters: GBS can be passed to the baby during delivery, potentially causing serious infections. If detected, antibiotics are given during labor to prevent transmission.

2. Non-Stress Test (NST)

• When: As needed, usually after 28 weeks.
• Purpose: Monitors the baby’s heart rate in response to movements to ensure the baby’s well-being.
• Why It matters: NST helps identify any signs of fetal distress, especially in high-risk pregnancies or if there are concerns about the baby’s growth.

3. Biophysical profile (BPP)

• When: Usually after 32 weeks, if indicated.
• Purpose: Combines an ultrasound with an NST to assess the baby’s movement, breathing, muscle tone, and amniotic fluid levels.
• Why It matters: This test evaluates the baby’s health and helps determine whether early delivery is necessary.

4. Growth ultrasound

• When: Around 32–36 weeks.
• Purpose: Measures the baby’s growth and checks the amniotic fluid levels and placenta position.
• Why It matters: Monitoring growth is critical for detecting issues like intrauterine growth restriction (IUGR) or macrosomia (large baby), which could impact the delivery plan.

Conclusion

Prenatal screening and diagnostic tests provide essential information for monitoring the health and development of your baby throughout pregnancy. Each trimester brings unique tests designed to ensure the baby’s well-being and help manage any potential complications. By understanding and following these tests, parents can make informed decisions about their pregnancy and prepare for a healthy delivery. For expert prenatal care and guidance, contact Shukan Hospital and IVF Centre today to schedule your tests and receive personalized support throughout your journey to a healthy delivery.

Author bio 

Dr. Ajay Prajapati

He is a highly respected specialist in the fields of infertility, IVF, and fetal medicine. With over a decade of hands-on experience, he has successfully treated numerous patients facing complex fertility and pregnancy-related challenges. His expertise in advanced laparoscopy has made him a trusted name for minimally invasive surgeries, providing patients with cutting-edge treatments and compassionate care.

As a FOGSI-certified sonologist, he ensures accurate and detailed assessments during pregnancies, particularly in high-risk cases. His commitment to excellence is reflected in the numerous successful outcomes and the trust he has earned from patients across Gujarat and beyond.

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