DOCTOR, is my baby okay? You would think this is the most commonly asked question of a radiologist during an obstetric scan. But the most commonly asked question is: “Is it a boy or a girl, doc?”

While I understand the importance of this question (think nursery bling), the obstetric scan is so much more. The radiologist is often the first person to see the developing baby. Ultrasound is performed during the early pregnancy for dating (assessing the age of the pregnancy), determination of the number of foetuses (developing babies), assessment of early complications and increasingly for evaluation of the foetus.

A wide range of abnormalities can be diagnosed with ultrasound between 11 and 14 weeks gestation (pregnancy age), including nuchal translucency abnormalities (abnormalities of the thickness of a fluid layer behind the neck), defects of the central nervous system, heart, anterior abdominal wall, urinary tract, and skeleton.

ANENCEPHALY

Anencephaly is a developmental defect of the central nervous system in which the brain and skull bones fail to form properly. It is the most common abnormality affecting the central nervous system and leads to foetal death, stillbirth or neonatal death (death within the first month of life).

The absence of skull bones may not be noticed and the head may appear normal. It is important to look for bone formation at the front of the head.

ENCEPHALOCELE

The term means herniation of structures in the skull through a defect in the skull due to either incomplete closure of the neural tube, which forms the central nervous system, or disruption by something called an amniotic band. Most cases occur at the back of the head.

SPINA BIFIDA

This is a birth defect where there is incomplete closure of the “backbone” and membranes around the spinal cord. The spinal cord and its covering membranes may then herniate through the defect to form a “meningomyelocele”.

Other central nervous system defects, including holoprosencephaly (failure of cleavage of both hemispheres of the brain) may also be detected in the first trimester.

FOETAL NASAL BONE

Down syndrome patients often have a flat face with a small nose. The absence of nasal bone formation at 11-14 weeks was found in 73 per cent of Down syndrome foetuses in a scientific study. Nasal bone evaluation may therefore be useful in screening for Down syndrome.

FOETAL NECK AND NT SCREENING

There is normally a thin layer of fluid in the posterior neck of a foetus in the first trimester of pregnancy. This layer is called the nuchal translucency and its thickness is measured with ultrasound.

By using a combination of maternal age, foetal NT thickness and certain blood tests to measure specific hormones produced in pregnancy, the detection rate for Down syndrome increases to 89 per cent. An increased NT thickness is also a marker for many structural abnormalities of the heart, bones and lungs as well as many genetic syndromes. An increased NT thickness is therefore an indication for detailed screening of the foetal anatomy.

FOETAL HEART

Abnormalities of the heart and great vessels are the most common congenital defects. Researchers have found that a complete examination of the heart can be successfully completed in 92 per cent of foetuses at 13 weeks gestation.

A wide spectrum of cardiac defects have been diagnosed in the first trimester, however, another examination at 18-20 weeks is still warranted in high-risk cases because of the small size of the heart in the first trimester and also the later appearance of certain structural and functional problems.

FOETAL ABDOMINAL WALL

There is a normal herniation of intra-abdominal contents at eight to 10 weeks. By 12 weeks, the intestine has returned to its normal position in the abdomen.

An omphalocele is a herniated sac with abdominal contents through a midline abdominal defect. The umbilical cord is attached to the apex of the sac. This differs from gastroschisis, where bowel loops herniate through an abdominal wall defect beside the umbilical cord insertion. A diagnosis of omphalocele should not be made before 12 weeks. Chromosomal abnormalities are often associated with omphalocele but rarely associated with gastroschisis.

Other abnormalities which may be detected include: urinary tract abnormalities of the kidneys, ureters and bladder such as multicystic kidneys, renal agenesis (absence of the kidneys) and megacystitis (large bladder); skeletal abnormalities such as shortening or absence of the limbs and clubfoot; and multiple gestations.

It should be stressed that an anomaly scan to look for abnormalities in foetal development is usually done in the second trimester between 18 and 22 weeks, therefore, many of the abnormalities detected on the first trimester ultrasound will almost certainly be reassessed as the foetus grows.

The important lesson to take home is that a baseline early ultrasound is extremely useful and can detect many serious foetal problems. Always try to plan your pregnancies and immediately begin prenatal care, which quite often will include a visit to your radiologist. To all the expectant mothers, have a safe pregnancy.

Dr Duane Chambers is a consultant radiologist and founding partner of Imaging and Intervention Associates located at shops 58 and 59 Kingston Mall, 8 Ocean Boulevard. He may be contacted through the office numbers 618-4346 or 967-7748.