Ultrasound in Pregnancy

An ultrasound is not 100% accurate. It will only find about 1 in 3 cardiac abnormalities and don’t always believe it when you are told the sex of the baby.

First Visit: An ultrasound is often performed at the first visit (6-12 weeks). Performing an ultrasound early in pregnancy serves a number of purposes. It can confirm that the pregnancy is okay and may detect early foetal abnormalities.

Although a urinary pregnancy test may detect a pregnancy even before you miss a period, an ultrasound will not see a sac within the uterus until at least five weeks. Approximately 1 in 5 pregnancies may miscarry so, if you wish to confirm that all is well, the best time to have an early ultrasound is seven weeks  from your last menstrual period or five weeks from conception.

The earliest thing seen on ultrasound is a small sac within the uterine cavity. A few days to a week later a small white ring, called the yolk sac, appears. Then approximately one-week later you’ll be able to see a small fuzzy white blur which represents the foetus and this will be attached to the yolk sac. A further week down the track and you will be able to see a foetal heart beating. If you are sure of your dates, then you should definitely see a foetal heart by eight weeks from the beginning of your last menstrual period.

Early Morphology (11-13 Weeks): Between 11 and 13 weeks an optional scan can be performed to assess the risk of your baby having a foetal abnormality such as Downs Syndrome.

Nuchal Translucency:  The nuchal skin fold is a small fluid-filled space in the fat pad at the back of the neck of all babies. An estimation of its thickness is called Nuchal Translucency [ N T ]. There has been a lot of investigation into the relationship between the thickness of this skin fold and the chance that the baby has an abnormality of its chromosomes.

Chromosomes determine our genetic structure. The normal number of these chromosomes is 46 and one of the most common chromosomal defects is Downs Syndrome in which there are three of number 21 chromosomes [i.e. Trisomy  21] in each cell making a total of 47 chromosomes instead of the normal 46.

It is known that all women have some risk of delivering a baby with this or other chromosomal defects. This risk increases as the woman gets older and is called the background risk. Research has shown that the chance of chromosome abnormality being present is increased when the NT increases. It also increases if there has been a past history of a chromosome abnormality, or if there is another abnormality noted on the ultrasound.

Most of this research has been performed in London. This research now has over 100,000 completed pregnancies which are able to be analysed. Using this data, it has been shown that between 70% and 80% of babies with chromosome abnormalities will be detected by this test (not 100%).

With any test like this there is a “False Positive” rate and for this particular test it is about 5%. This means that about 5 women in 100 will have a positive NT Test but the baby’s chromosomes will be normal.

The test calculates your individual adjusted risk for this pregnancy. It is important to understand these figures. “One in” means that no matter how low the result is, Downs Syndrome will occasionally occur. Also, approximately 1 out of 4 babies with Downs Syndrome will not have a thick N T. and so will not be detected by this test. It is also important to understand that this test is only looking for chromosomal abnormality, although it is probable that some babies with major heart defects may be identified. Spina bifida, face defects, kidney defects, etc. are not likely to be detected by an ultrasound at this early stage.

It is for these reasons that a further assessment of a baby is suggested by an ultrasound later in pregnancy [18-19 weeks]. A blood test can be combined with this early ultrasound to increase the pick-up rate. This looks at the level of 2 hormones, PAP A and HCG, in maternal blood and these levels are added to an algorithm to give an overall risk assessment.

Morphology Ultrasound (18-20 Weeks): The Morphology Scan is a fairly exhaustive study of the baby and its environment, looking at the babies structure as well as the placenta, uterus and fluid surrounding the baby. By taking measurements of the baby it is possible to estimate the babies age [in weeks] and this is accurate to within 7 -10 days at this time. The placenta can be examined to make sure that it is not covering the cervix or lying too close to it [Placenta Praevia]. The baby / babies can be examined and we look for structural abnormalities of the brain, heart, kidneys, spine, limbs and internal organs. We also look for evidence of soft signs [markers] of chromosomal abnormalities.

Growth Scan:
If your baby/babies do not appear to be growing well, then further ultrasounds may be performed at intervals throughout the pregnancy as required to check health and well-being. An ultrasound to estimate the weight at delivery has an inaccuracy of about 10-20% of the actual birth weight. For those with a low lying placenta on the Morphology scan, there is a small risk of this persisting as a Placenta Praevia. A follow-up san is performed around 30 – 34 weeks to make sure the placenta has moved.

Estimated Foetal Weight: If you have had a previous caesarean section or are having a big baby, an USS at 36 weeks will give an estimate of the babies weight. Doctors monitor growth throughout the pregnancy using ultrasound and clinical experience to give you the best estimate I can of your babies size.