Most women will have an ultrasound examination of their baby at about 20 weeks gestation. This is because at this gestation the fetus is fully formed and also large enough to see the organs and structures in detail. It is important to understand why this test is performed and what it tells us.
The scan will provide information on the following:
- the expected date of delivery (term date)
- fetal growth and movement
- presence and absence of fetal abnormalities
- placental site
- cervical length ( to predict the risk of very early delivery)
- the uterus and ovaries
It is known that 4% ( 1 in 20) babies will have some sort of abnormality. These may be minor but some will be more serious, for example abnormalities of the fetal heart, brain or spinal cord. If there is a history of a fetal abnormality in your family it is important to tell the doctor so this can be checked.
Limitations of the scan
Unfortunately not all abnormalities can be diagnosed before birth.
This is because
- they occur after the time of the scan
- they are too small to be detected
- the scan is only ever a picture of the fetus and does not necessarily indicate function
- inadequate views of the fetus because of the fetal position or if the mother is overweight
Despite these potential limitations, if your scan is reported as normal, the chance of an unrecognised problem before birth is very low.
Ultrasound uses sound waves and not radiation. It has been used in pregnancy for over 30 years and to date no harm to mothers or fetuses has been reported.
3 D and 4D Ultrasound
There are a few medical reasons for a 3D or 4 D scan but these are usually performed for social reasons. A referral is not needed for these scans as they do not attract a medicare rebate.