backup og meta

How to Diagnose Birth Defects through Congenital Anomaly Screening

Medically reviewed by Ann Guevarra MD, OB-GYN Diplomate, POGS · Obstetrics and Gynecology

Written by Fiel Tugade · Updated Aug 10, 2022

How to Diagnose Birth Defects through Congenital Anomaly Screening

As a pregnant woman, check-ups and doctor’s visits over the past weeks are as necessary as they can be. But during your 20th week, your doctor recommends that you undergo a congenital anomaly screening. Some of your mom friends say that it is a vital scan you need to take on, but what is it? Why is it important and how can it help diagnose birth defects during pregnancy? Find out here. 

Congenital Anomaly, Defined

Congenital anomaly, or in simple terms birth defect, refers to any abnormality that occurs during pregnancy. It can be structural, functional, genetic, chromosomal, or biochemical in nature, and it can be minor or major.

Such abnormalities are the leading causes of disabilities, chronic illnesses, and even childhood deaths. 

Other terms for this are congenital malformations, congenital disorders, or congenital abnormalities.

Depending on the type of birth defect, doctors are able to diagnose it during pregnancy or after the baby is born. They can detect it through congenital anomaly screening tests like ultrasound, amniocentesis, or chronic villus sampling (CVS).

While the prenatal diagnosis may not always treat the anomaly, it can assist parents in preparing for having a child with a birth defect.

Types of Congenital Anomalies

There are two broad classifications of congenital anomalies – structural and functional. 

Structural anomalies. These birth defects affect how the body is built. Some newborns may have a body part that is missing or improperly formed. 

Abnormalities that fall under this category are:

  • Spina bifida
  • Hypospadias
  • Cleft lip 
  • Ventricular septal defect

Functional anomalies. These affect how the body works and develops as a system. 

Some common types bring an effect on the brain (or the nervous system) like:

  • Down syndrome
  • Phenylketonuria
  • Hemophilia
  • Tay-Sachs disease

Most women undergo some form of congenital anomaly screening such as a blood test. This will helps determine the risk of having a baby with a birth defect, like Down syndrome and spina bifida. 

Causes and Risk Factors of Congenital Anomalies

Although nearly half of all congenital abnormalities cannot be linked to a specific cause, some considerable causes and risk factors include:

  • Genetics
  • Socioeconomic and demographic factors (inadequate diet and nutrition)
  • Environmental factors
  • Infections (i.e., syphilis, rubella, Zika virus)
  • Drugs and Alcohol
  • Maternal risk factors and conditions (i.e., age, obesity, pre-gestational diabetes)

Some women opt to have a preconception examination to ensure a healthy pregnancy. Through this, the doctor will evaluate your overall health and identify any risk factors that may affect the pregnancy. 

What is a Congenital Anomaly Screening?

Congenital anomaly screening refers to a detailed ultrasound scan (mid-pregnancy or anomaly scan) that examines the baby’s physical development. It takes a closer look into the formation of the baby’s different body parts and organs. Some of which include:

  • Bones
  • Heart
  • Brain
  • Spinal cord
  • Kidney
  • Abdomen
  • Face

However, it cannot detect every condition there is. 

Some people commonly refer to this as the 20-week screening scan as it usually takes place between the 18th and 21st week of pregnancy.

Similar to the 12-week screening scan, it generates a 2D black-and-white image of the baby from the scan.

Diagnosis of Birth Defects During Pregnancy

During pregnancy, your doctor may suggest some screening tests to check for birth defects or other issues that may affect the woman or her baby. Your doctor may run several tests to screen and evaluate the likelihood of any birth defect. 

Combined First Trimester Screening (CFTS)

This is a maternal serum screening test that typically takes place before 12 weeks of pregnancy.  It uses ultrasound and serum markers to predict the likelihood of certain chromosomal anomalies:

  • Patau syndrome (Trisomy 13)
  • Edwards syndrome (Trisomy 18)
  • Down syndrome (Trisomy 21)

Non-invasive Prenatal Testing (NIPT)

This type of congenital anomaly screening takes out a blood sample from the mother to assess the probability of such abnormalities. In addition that, it can also detect some sex chromosome abnormalities. 

Second Trimester Maternal Serum Testing (2TMSS)

This single blood test is performed between 14 and 20 weeks of pregnancy. It helps in estimating the chances of having chromosomal anomalies and neural tube defects.


An ultrasound produces images of the baby. The first-trimester ultrasound screen looks for extra fluid behind the baby’s neck, a procedure known as the nuchal translucency scan. If the ultrasound reveals an increase in fluid, the baby may have a chromosomal disorder or a heart defect.


This is a congenital anomaly screening test that collects a small portion of the amniotic fluid from the area surrounding the baby. The fluid goes through laboratory analysis to determine the protein levels in the baby, which indicates some birth defects. 

Chorionic Villus Sampling (CVS)

This refers to a test wherein the doctor takes out a small sample of the placenta (chorionic villus). 

Doctors usually offer this form of congenital anomaly screening to women who had an abnormal CFTS result. Those who have greater risks may also go through CVS. Such risks include being 35 years or older by the baby’s due date, having a family history or known genetic disease, possible exposure to teratogens (e.g. radiation, alcohol, certain drugs), and a history of abnormal ultrasound findings. 

Key Takeaway

Being pregnant requires not only your health but more so the health of the child as they grow in your womb. 

If you are worried about the possibility of any birth defect, you may discuss it with your obstetrician first. The doctor will tell you if you need to undergo a congenital anomaly screening or not.

Learn more about Prenatal Care here.


Hello Health Group does not provide medical advice, diagnosis or treatment.

Medically reviewed by

Ann Guevarra MD, OB-GYN Diplomate, POGS

Obstetrics and Gynecology

Written by Fiel Tugade · Updated Aug 10, 2022

ad iconadvertisement

Was this article helpful?

ad iconadvertisement
ad iconadvertisement