What Is A Pregnancy Scan?

This information has been written in an attempt to inform you of the benefits and limitations of an ultrasound pregnancy scan.

What is an ultrasound?

The scanning technique uses high frequency sound waves, which cannot be heard by the human ear, to produce images of the pregnancy. It is used to obtain valuable information about your pregnancy.  Ultrasound has been routinely used in pregnancy for over 35 years and there is no evidence to date to suggest that it is harmful to baby.

Why have an anatomy scan (routine pregnancy scan) ?

There are many valid medical reasons for your midwife or doctor to request a scan.  These include:

–         To assist in estimating when your baby is due

–         To demonstrate the heartbeat

–         To identify the number of babies: one, two, three or more!

–         To determine the position of the placenta (afterbirth)

–         To check the amniotic fluid around the baby

–         To monitor the growth of the baby if there is concern about the size

–         To look at the anatomy of the baby

When is a scan performed?

A scan can be performed at any stage during a pregnancy.  In early pregnancy a scan may be indicated if there is any bleeding or pain or a history of previous pregnancy problems.  If a scan has been performed in the first 14 weeks, your due date will hopefully have been clarified at this stage.  The early scan (less than 12 weeks) can usually be performed abdominally but, on occasion, a vaginal (internal) scan may be necessary, to obtain as much information as possible.  A routine anatomy scan is usually performed between 18-20 weeks.  (If you have had an earlier scan performed in the first 14 weeks we may well try and time your anatomy scan for 21-22 weeks.

Can you tell whether it is a boy or a girl?

The scan can sometimes tell what sex the baby appears to be, but not always.  Determining the sex of the baby is not always easy on a scan and it depends on the baby lying in a certain position to visualise the genital area and the baby does not always oblige!

Do I need a full bladder for the routine scan?

For early pregnancy it is necessary to have a full bladder.  At 18 weeks gestation onwards it is not, unless you have been specifically advised to attend with a full bladder.

Will an anatomy scan detect the abnormalities of the baby?

The majority of babies are healthy at birth, but unfortunately about 2-3 % of babies are born with a birth defect.  About half of these major defects will be seen on scan and half will not be detected during pregnancy.  This means

that even if your scan is apparently normal there is still a small chance that your baby may still have a problem because the ultrasound has its limitations.

The following table lists different types of birth defects and how likely scanning is to identify each problem during pregnancy:

Spina Bifida Open Spinal Cord 90%
Anecephaly Absence of the top of the head 99%
Hydrocephalus Excess fluid within the brain * 60%
Major Heart Problems 20%
Diaphragmatic Hernia A defect in the muscle which seperates the chest and the abdomen 60%
Exomphalos/Gastoschisis Defects of the abdominal wall 90%
Major Kidney Problems Missing or abnormal kidneys 85%
Major Limb Abnormalities Missing bones or very short limbs 15%
Cerebral Palsy Spasticity Not Seen
Autism Not Seen
Down’s Syndrome May be associated with heart and bowel problems About 40%

*many cases present for the first time late in pregnancy or even after birth.  The above information is based on audit figures of own practice at the National Maternity Hospital.

The vast majority of babies will be normal.  If at the time of a scan a problem is suspected, you will be told at that stage that there may be a problem.  A full discussion of the problem may require you to come back to for a further scan and discussion with a fetal medicine specialist.  This can usually be organised within  24 hours if the problem is deemed urgent.  Most problems that need repeat scanning are not serious and approximately 15% of scan will need to be repeated for one reason or another.

Can I have a photograph of the baby?

Yes, of course, but again this depends on the baby being in the right position to get a good picture.