Case Study Reports: Antenatal Screening

ANTENATAL SCREENING

History

A woman aged 23 years is referred by her general practitioner to the antenatal clinic at 14 weeks in her first pregnancy. She has booked late having only just discovered she is pregnant. Although the pregnancy was unplanned she is now happy about it. She sepa- rated from her partner of 2 years a few weeks ago but is supported by her family and friends. She has no significant medical history and is one of four siblings. On direct ques- tioning her mother apparently had a stillbirth attributed to some form of congenital abnormality 28 years ago. Otherwise the pregnancy is assessed to be low risk.

As she is too late for a nuchal translucency test she is offered serum screening for Down’s syndrome, which she agrees to. This is performed at 17 weeks.

Questions

·             How should she be counselled and what further options are available?

·              If the pregnancy is not affected with a fetal abnormality, how else could this test result be explained?

Answers:

AFP, oestriol and β-HCG are used in combination in the ‘triple test’, one of the serum- screening tests available to detect Down’s syndrome. The results are expressed as mul- tiples of the median (MoM) and in this case the AFP is significantly raised.

Down’s syndrome is associated with decreased AFP, increased oestriol and increased

β-HCG.

Increased AFP is associated with an increased risk of fetal abnormality, the commonest being:

·              neural tube defects

·              anterior abdominal wall defects

·              Patau’s syndrome (trisomy 13).

The woman should be counselled that the risk of Down’s syndrome is low but that the blood test suggests a possible increased risk of the other abnormalities described.

She should be referred to a specialist fetal medicine centre for a detailed ultrasound scan to exclude any significant neural tube or abdominal wall defects. Depending on the ultrasound result, she may wish to terminate the pregnancy if a serious abnormality is detected, or to continue but with appropriate geneticist and paediatric input antenatally to prepare for the delivery.

One of the problems with serum screening is that other pregnancy complications may lead to a higher level of AFP. These include:

·              multiple pregnancy

·              fetal intrauterine growth restriction

·              oligohydramnios.

AFP is also affected by ethnic background, maternal diabetes and maternal liver disease. Finally the normal range is dependent on gestation, and unless the gestational age has been confirmed, the AFP may be high because the pregnancy is more advanced than presumed.