11-13-week antenatal scan

11-13-week antenatal scan

11-13-week antenatal scan (Nuchal translucency) scan is considered a routine scan advised for the well-being of fetal as well as for early pregnancy screening.

What does it include?

It is highly dependent on the operator and includes multiple components. Traditionally three factors are used to calculate the risk of trisomy.

CRL – Crown-rump length = must be 45-48 mm, gestational 11 weeks to 13 weeks

Nuchal translucency

FHR – Fetal heart rate

Additional markers may increase the detection rate and reduce false-positive rates:

  • Nasal bone
  • Ductus venous flow
  • Tricuspid flow

By combining these factors with blood tests (i.e., dual marker) has been reported to achieve a nearly 95% detection rate for trisomies.

Apart from the scanning protocol, early detection of the major abnormalities may be possible, and the checklist must include the following conditions:

Acrania

It is a rare lethal congenital abnormality characterized by an absence of calvaria.

Epidemiology– The estimated incidence is at – 1: 1000 pregnancies.

Pathology– The condition can be thought to result from abnormal migration of mesenchymal tissue, which generally covers the cerebral hemispheres. The calvarial bones are partially or entirely absent with related (albeit abnormal) preservation of growth of the cerebral hemispheres. A thin membrane surrounds the cerebral hemispheres.

Radiological features-

Antenatal ultrasound- The fetal cranium ossifications begin and accelerates at 9 weeks, antenatal ultrasound allows diagnosis from 11 weeks onwards. It is a must to see the specifications for frontal bone ossification in the coronal and axial planes.

Treatment and prognosis- Acrania is lethal and may progress to anencephaly through the acrania-anencephaly sequence.

Differential diagnosis-

Imaging differential considerations conditions include:

  • Meroacrania – it is the preservation of the occipital bone
  • Anencephaly/exencephaly – deficiency/absence of the brain tissue as well.
  • Congenital hypophosphatasia
  • Achondrogenesis
  • Severe osteogenesis imperfecta- additional findings such as shortening, bowing or long bone fractures often aid in distinction from acrania.
  • Calvarial defects due to an amniotic band.

Encephalocele-

It is also known as meningoencephalocele, it is a type of neural tube defect and a form of cephalocele where brain tissue and overlying meninges herniate out through a defect in the cranium.

Terminology

It ought to be recognized from cranial meningocele in which the lesions contain just meninges, and from extracranial brain herniation in which the brain herniates through Dural and skull defects (for example post-injury or medical procedure).

Classification

A number of classification systems exist for encephaloceles, many of them are only of subspecialty interest, and most use very similar terminology, sometimes with different meanings. For example, the term nasal encephalocele is variably used, often to encompass both frontoethmoidal and basal encephaloceles.

Epidemiology

In one of every 4000 live births, they are thought to occur. Frontoethmoidal encephaloceles are more common in southeast and south Asian populations and there is no recognized gender predilection.

Clinical presentation

Frontoethmoidal encephalocele manifest as a clinically visualized mass along with the nose. The intracranial root of most frontoethmoidal encephaloceles present at the foramen cecum (a small ostium located at the lower end of a small depression anterior to the crista Galli) and formed by termination of frontal bone and ethmoid bone.

Basal encephaloceles are internal and generally are not externally visible, although they can manifest as a lump or bump in the oropharynx or nasopharynx.

Pathology

Congenital encephalocele is the most common and is a form of neural tube defect and occurs due to a defect in fusion during the process of embryogenesis of the cartilaginous neurocranium, the membranous neurocranium, or viscerocranium.

In adults, encephalocele is mostly caused due to injury but is also increasingly recognized in idiopathic intracranial hypertension.

Radiographic features

An encephalocele can be seen as a purely cystic mass or may contain echoes from herniated brain tissue. Encephaloceles that are bigger in size mat show accompanying microcephaly.

Pathology

Similarly, as with a wide range of holoprosencephaly, it is an uncommon congenital brain malformation wherein there is a failure of the complete partition of the two halves of the globe (which as a rule happens around the fourth to sixth gestational weeks) and disappointment of cross over cleavage into the diencephalon and telencephalon.

Radiographic features

Similarly, as with most cerebral structural congenital anomalies, alobar holoprosencephaly is visible on each modality, yet, as a rule, is recognized on antenatal ultrasound (whenever performed), and best described by MRI.

Iniencephaly

Iniencephaly is a rare neural tube defect resulting in the features include:

  • Occipital bone defect
  • Partial or total rachischisis (absence) of cervicothoracic vertebrae.
  • Fixed fetal head retroflection

Epidemiology

The estimated incidence is 0.1-10 per 10,000 births (life). They are recognized female predilections.

Pathology

The exact etiology is not known.

Radiographic features

  • Antenatal ultrasound

Features include

  • Exaggerated cervicothoracic lordosis
  • deficient (short) or fused cervical vertebrae
  • variable deficit in the occipital bone
  • due to the position of the fetus head, crown-rump length can be lower than expected for gestational age.
  • Can have concurrent polyhydramnios

Gastroschisis

Gastroschisis alludes to an extra-abdominal herniation (destruction) of fetal or neonatal bowel loops (and at times segments of the stomach as well as liver) into the amniotic cavity through a para-umbilical anterior abdominal wall.

Epidemiology

The estimated incidence is at approx. 1-6 per 10,000 live births. There may be a male predilection and a higher incidence with younger maternal age.

Pathology

This anomaly doesn’t have a surrounding membrane (in contrast to a simple omphalocele). It is the small bowel that herniates frequently. The defect is constantly on the right side and typically measures between 2-4 cm. There is no covering membrane or membrane remainder.

Content

The small intestine consistently herniates through the abdominal wall defect and lacks normal rotation and fixation to the posterior abdominal wall. Notwithstanding the small intestine, the large intestine, stomach, parts of the genitourinary system, and liver may herniate through the imperfection also.

Etiology

A trade-off in vascular supply to the area in the abdominal wall adjacent nearby the umbilicus might be a causative factor. Some additionally propose a deficient relapse of the correct umbilical vein as a possible causative factor.

Radiographic features

Antenatal ultrasound

In most cases it is noted that the herniated content is towards the right side of the umbilical cord; a color doppler can be useful to locate the cord in relaxation to the herniation. This causes the fetal abdominal circumference to be smaller than expected for GA (gestational age). The herniated bowel often shows free-floating rather than contained. The herniated bowel wall may be thickened due to a hernia.

Omphalocele

Also known as exomphalos- are defined as congenital midline abdominal wall defects at the base of the umbilical cord insertion along with herniation of out of the fetal abdomen.

Epidemiology

The estimated occurrence may be up to 1:4000 live births.

Pathology

Typical a greater anterior abdominal wall defect will be present, through which the liver or/and bowel herniate, covered by a peritoneal membrane as well as amnion. The umbilical cord typically inserts at or near the apex.