Role of Ultrasound in Assessing Fetal Anomalies

Introduction

Ultrasound is indicated in each trimester of pregnancy for evaluating the condition of both mother and fetus. The timing and frequency of the ultrasound are based on the maternal and fetal risk factors for anomalies. The early diagnosis of fetal anomalies helps to initiate a proper management plan. There is a high risk of adverse fetal outcomes like stillbirth due to the increased incidence of hypertension, gestational diabetes, and morbid obesity in the reproductive age group. There are specific indications that require screening in every trimester.

Three-dimensional and four-dimensional ultrasound is used along with two-dimensional ultrasound in the diagnosis of fetal abnormalities is in the process of development and can be used in the future. Genotypic and phenotypic links are known for numerous genetic syndromes. Other invasive methods that are more accurate and safer are performed along with ultrasound guidance for a better diagnosis.

What Is the Role of Ultrasound in Assessing Fetal Anomalies?

The indications for ultrasound are based on the trimester that must be met before performing the exam:

• Level I: It is the standard scan done in the second trimester to perform a simple fetal anatomic survey examination in low-risk pregnancies.

• Level II: It is a specialized second-trimester anatomy ultrasound or targeted examination, then abnormalities in the standard anatomy examination or in pregnancies at high risk for anatomic abnormalities.

Various other examinations include

• Fetal echocardiogram.

• Fetal doppler ultrasound.

• Cervical length measurement.

• Biophysical profile.

Fetal ultrasound is a regular and standard method that is advised in every pregnancy. The aim is to examine the high-risk population and detect abnormalities in the early stages. Conventionally, a complete fetal anatomic survey is done in the mid-second trimester to assess the anatomic anomalies.

Abnormalities can be detected in the late first trimester, owing to improvement and advancement in ultrasonography. Early detection of fetal anomalies helps the patient to perform genetic counseling, further diagnostic testing, and sufficient time to make an informed decision.

When Is the Ultrasound Performed for Accessing Fetal Anomalies?

1) Standard First-Trimester Ultrasound Examination: It is indicated to confirm a singleton or multiple gestation intrauterine pregnancy. This ultrasound helps in detecting the following:

• Confirmation of cardiac activity.

• Estimation of gestational age.

• Evaluation of suspected ectopic pregnancy (implantation of the fertilized egg outside of the womb, usually in the fallopian tubes).

• Measurement of nuchal translucency (measures the nuchal fold thickness) for aneuploidy screening.

• Pelvic pain.

• Vaginal bleeding.

• Suspected gestational trophoblastic disease.

• Maternal pelvic organ assessment.

2) Early Comprehensive Fetal Anatomy Ultrasound or Detailed First-Trimester Anatomy Ultrasound Examination: It is a special diagnostic examination for women at increased risk of placental and fetal anomalies performed between 12 weeks and 0 days to 13 weeks and six days. The ideal time for performing an early anatomy scan is after 13 weeks. This ultrasound does not replace a detailed second-trimester anatomic ultrasound that must be performed. It is either performed transabdominally or combined with a transvaginal ultrasound. Indications for this ultrasound include:

• A history of a previous child with an anomaly.

• Increased nuchal translucency of more than three millimeters.

• A suspected fetal anomaly in the current pregnancy.

• Pre-gestational diabetes mellitus.

• Advanced maternal age.

• Congenital infection exposures.

• Maternal obesity.

• In vitro fertilization (IVF) pregnancy.

• Abnormal placental implantation.

• Teratogen (an agent that causes malformation of an embryo) exposure.

• Multifetal gestation.

• Positive maternal serum screening tests.

3) Standard Second-Trimester Anatomy Ultrasound Examination: It is a routine anatomy ultrasound that is performed between 18 to 22 weeks in every pregnancy to assess the following:

• Fetal anatomy.

• Screen for anomalies.

• Evaluate the placental location.

• Maternal pelvic anatomy.

• Estimate gestational age if earlier scans are not performed.

The standard anatomical ultrasound is done in low-risk pregnancies where a detailed anatomy scan is not indicated.

4) Detailed Second Trimester Anatomy Ultrasound Examination: This scan is an indication-driven examination. It is not necessarily performed in all pregnancies. The indications include

• A previous child with an anomaly.

• Increased nuchal translucency of more than three millimeters.

• A suspected fetal anomaly in the current pregnancy.

• Advanced maternal age.

• Pregestational diabetes mellitus.

• Early gestational diabetes before 24 weeks.

• In vitro fertilization (IVF) pregnancy.

• Multifetal gestation.

• Maternal obesity.

• Teratogen exposure.

• Congenital infection exposures.

• Positive maternal serum screening tests.

• Abnormal placental implantation.

• Aneuploidy marker on previous ultrasound.

• Parental chromosomal or genetic carrier.

• Alloimmunization (an immune response occurring after exposure to genetically different cells or tissues).

• Polyhydramnios (excessive amniotic fluid).

• Oligohydramnios (low levels of amniotic fluid).

• Maternal drug use.

5) Specialized Third-Trimester Ultrasound Examination: This is an indication-driven examination scan. It includes the following types of specialized ultrasounds

• Biophysical profile.

• Additional biometric measurement follow-ups (every three to four weeks).

• Doppler ultrasounds in the case of growth restrictions.

Indications of this scan include:

• Fetal growth restriction or suspected macrosomia.

• Multifetal gestation.

• Twin-twin transfusion syndrome screening (every two weeks).

• A significant discrepancy between the uterine size and clinical dates.

• Evaluation of pre-labor rupture of membranes.

• Evaluation of fetal well-being.

• Preterm labor.

• Vaginal bleeding.

• Suspected placental abruption.

• Suspected amniotic fluid abnormalities.

• Follow-up evaluation for placental appearance for suspected placenta previa.

• Placenta accreta spectrum or PAS (a term for various pregnancy complications due to abnormal placental implantation).

• Vasa previa.

• Suspected fetal death.

What Are the Contraindications of Ultrasound During Pregnancy?

Patient refusal is the absolute contraindication of performing fetal ultrasound. Ultrasound is usually a safe diagnosis method during pregnancy and must only be used when done with the doctor’s advice under the as low as reasonably achievable (ALARA) principle. M-mode imaging can be used for monitoring cardiac activity, and pulse doppler must be discouraged if it will not provide any valuable information to the examination.

Conclusion

The major role of ultrasound is to diagnose fetal genetic syndromes. It acts as a screening tool for the identification of placental, fetal, and amniotic abnormalities. The knowledge of patterns seen in individual syndromes helps in the identification of these abnormalities with definitive diagnostic testing and preparation for the post-natal period. Recent evidence assists in the application of first-trimester ultrasound as a screening tool for certain genetic syndromes. Second-trimester ultrasound is extremely essential as it gives the most accurate information about the fetal abnormalities. Therefore, it must be performed in a systematic and structured manner.