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Fetal Anomaly Scan

Second trimester scan is a routine follow – up scan for every pregnant women during their 15 weeks to 27 weeks of pregnancy. During this time of pregnancy, they are offered to do “Detailed Scan” which is also known as Fetal Anomaly Scan.

The aim of the detailed scan is to look for the baby's anatomy in detail and try to find out  developmental and structural abnormalities of the baby, and to determine the position of the placenta. The scan will detect 80% of babies who have serious physical abnormalities, for examples spina bifida, cleft lip, anencephaly, hole in the heart and omphalocele. The best time to do this scan is at 18 to 22 weeks of pregnancy. This is because the baby's parts are not crowded , liquor is adequate and most of the fetal part can easily be seen. The scan usually takes about 30 minutes and will depend on the baby’s position, movement and liquor status . A longer time may be taken (~ 45 minutes) when the  baby is facing downward, moving excessively or has decreased liquor volume.

During detail scanning, the following structures of the baby will be identified and look for abnormality.

  • Fetal lie
  • Placenta localization
  • Amniotic fluid
  • Cord Insertion and vessels
  • Head
  • Face
  • Neck
  • Heart
  • Lungs, Diaphragm
  • Abdomen (stomach, kidneys , intestine, liver , bladder)
  • Genitalia
  • Limbs (upper and lower limbs) and extremities
  • Spine
  • Cervix of the mother

Advantages of Doing Detailed Scan

The scan will show fetus’s anatomy including the lips, nose, legs, feet, heart, lungs, stomach, kidneys, bladders, umbilical cord and placenta. Any major abnormalities in these parts can be detected during the detail scanning. The liquor volume will be assessed at the same time. Excessive or decreased of liquor volume is commonly associated with fetus anomalities. Example, polyhydramnios is found in spina bifida, anencephaly, esophageal atresia. This is also a good time to identify the position of the placenta to ensure that it is not covering the cervical os.

Some of the structured abnormalities of the newborn are life – threatening at the time of birth, example diaphragmatic hernia, esophageal atresia. Knowledge of this condition before birth will allow the obstetrician to prepare and manage the fetal abnormality soon after birth.

What will the Medical Sonographer look at ?

  • The shape and structures of the fetal head
  • The fetal facial profile and face  to rule out any cleft lip/cleft palate
  • The fetal spine in both longitudinal (length) and transverse (cross section) view, to make sure that there are no spinal defect.
  • The fetal abdominal wall for defects such as omphalocele or gastrochisis, the stomach, intestine and fetal ascites.
  • The fetal kidneys to ensure that the fetus have two kidneys and functioning, and the bladder that fills and empty throughout the scanning process.
  • The fetal upper and lower limbs (hands,fingers, legs, feet, toes) to look for polydactaly and talipes.
  • The heart to look for its position and size as well as chamber size, outflow defect and septal defect.
  • The umbilical cord for hernia, cyst and the normal 2 arteries and 1 vein.
  •  The placenta for its position, size and abnormal mass.
  • Detail Scan also monitor the growth of the baby using bi-parietal diameter, abdominal circumference and femur length.

 What are the abnormalities that can be detected?

The table below shows the common abnormalities that can be detected and the chances of being seen in detail scan.

Problem
What the problem is
Chance of being seen
Spina bifida Open spinal cord 90%
Anencephaly Absence of the top of the head 99%
Hydrocephalus *Excess fluid within the brain 60%
Major congenital heart problems   25%
Diaphragmatic hernia A defect in the muscle which separates the chest and abdomen 60%
Exomphalos/ gastroschisis Defects of the abdominal wall 90%
Major kidney problems Missing or abnormal kidneys 85%
Major limb abnormalities Missing bones or very short limbs 90%
Cerebral palsy Spasticity Never seen
Autism   Never seen
Down Syndrome May be associated with heart and bowel problems About 40%

Source: Royal College of Obstetricians and Gynaecologists (2000)

About 20% of all abnormalities will not be seen in the fetal anomaly scan because these abnormalities are difficult to detect or they appear at the later part of your pregnancy. So, there is a small chance that your baby may have abnormalities at birth which are not detected during fetal anomaly scan.

Detail Scan of different regions during pregnancy

Fetal Lie and Presentation

During the second trimester scan, sonographer will check for fetal’s position and presentation:

  • Cephalic
  • Breech
  • Transverse

Placenta Localization

  • To rule out placenta previa (low lying placenta) , position and to check for  placental mass and placenta calcification.
  • Placenta location : The normal location are anterior, posterior or fundal placenta
  • The abnormal location of the placenta is reffered to as placenta previa. In placenta previa, the distance of placenta edge to internal os is less than 2.5 cm. In extreme cases the placenta is right over the internal os,
  • Placenta previa is classified according to its location in relation to internal os :
    • Type I or low lying: The placenta edge reaches the lower segment of the uterus but does not infringe on the internal os.
    • Type II or marginal: The placenta edge touches, but does not cover the internal os
    • Type III or partial: The placenta edge partially covers the internal os
    • Type IV or complete: The placenta edge completely covers or sits over the internal os

 

 
Placental-anterior upper segment
 
Placental-posterior
     
   
Placental-fundal
   

Fetal Head Parameters

The biparietal diameter (BPD) and head circumference (HC) are the most accurate second trimester measurement for gestational age. These two parameters can be used to determine gestational age with a 95% confidence ( 14 days) and these are used to predict the expected date of delivery.

 

This image shows transverse head where measurements such as Biparietal Diameter (BPD) and Head Circumference (HC) are measured

 

Ultrasound view of the normal brain structures such as choroid plexus and lateral ventricles

     
   
Posterior fossa view of normal fetal head, which includes cerebellum, cisterna magna and nuchal pad.
   

Fetal Face (Facial Profile)

It is important to get a good image of a fetal profile and the face which show the nasal bone, nose, lips as well as the chin. This is obtained at the sagittal view. In coronal view the fetal orbits, nose, lips and palate can be seen.

This photo shows a baby's side view, forehead, nose, lips and chin

     
 
This image shows fetal orbital (eyes)

This image shows the nose and lips of the baby

Fetal spine and abdominal wall

It is crucial to check on baby's spine in both the longitudinal view and in cross section, moving through the neck, the shoulder and to the pelvis. This is to ensure that all the vertebrae are in alignment, and the skin covering the spine on the back and the abdominal wall are intact. Deficiency in the skin covering can occur in spina bifida, gastrochisis and omphalocele.

 
   
Abdomen
 
Spine
   
     

Cord Insertion at the umbilicus

Fetal kidneys and bladder

The two kidneys in the fetus lies on either side of the spine, and are below the stomach. The kidney seen as a “bean shape” and outer part of the kidney is the cortex and innermost is the renal pelvis.  If the baby's bladder is full, it's easy to see as a black bubble in the pelvis.

Fetal kidneys (left and middle images) & Fetal bladder – umbilical arteries view (right images)

Fetal heart, stomach and diaphragm

One of the main component in fetal detail scan is to examine the fetal heart. The top two chambers of the heart are called  the 'atria', and the bottom two chambers are called the 'ventricles'. The two atriums should be equal in size and the two ventricles should be equal in size. The valves between the atria and ventricles should open and close with each heartbeat.

Beneath the heart is the diaphragm and below the diaphragm  is the stomach. The fetus regularly swallows some of the amniotic fluid in which it lies.  This fluid that the fetus swallowed can be seen in the stomach as a black bubble.

 

Four chamber view of fetal heart

 
Fetal Heart Rate
     
   
Fetus diaphragm that separate the thoracic and abdomen cavity (arrow in left image)
 
Diaphragm (arrow pointed)
 
Stomach (arrow pointed)

Fetal long bones, hands and feet

The long bones of the arms and legs,as well as the position of hands and feet are examined during detail scan. The fingers and toes are examined to detect any abnormalities such as polydactaly, fingers webbing and clubfoot.

 
Fetal arm and hand
 
Fetal leg and feet

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